Li/na-ion battery anode materials

ABSTRACT

The invention relates to active electrode materials and to methods for the manufacture of active electrode materials. Such materials are of interest as active electrode materials in lithium-ion or sodium-ion batteries. The invention provides an active electrode material expressed by the general formula [M][Nb] y [O] z ; wherein the active electrode material is oxygen deficient; wherein M consists of one of Mg, Cr, W, Mo, Cu, Ga, Ge, Ca, K, Ni, Co, Al, Sn, Mn, Ce, Sb, Y, La, Hf, Ta, Zn, In, or Cd; y satisfies 0.5≤y≤49; and z satisfies 4≤z≤124.

FIELD OF THE INVENTION

The present invention relates to electrode active materials and tomethods for the manufacture of electrode active materials. Suchmaterials are of interest for example as electrode active materials inlithium ion or sodium ion batteries.

BACKGROUND

Lithium ion (Li-ion) batteries are a commonly used type of rechargeablebattery with a global market estimated at $40bn in 2018 and predicted togrow to $200bn by 2030. This large market is divided between variousapplications, ranging from transport and utility-scale energy storage toconsumer electronics and power tools. Accordingly, rechargeable(secondary) Li-ion batteries are currently under intense research anddevelopment to improve their performance to reach industrial demands ofthe technology [Goodenough and Park (2013)]. In particular, Li-ionbatteries are the technology of choice for electric vehicles that havemultiple demands across technical performance to environmental impact,providing a viable pathway for a green automotive industry.

A typical lithium-ion battery is composed of multiple cells connected inseries or in parallel. Each individual cell is usually composed of ananode (negative polarity electrode) and a cathode (positive polarityelectrode), separated by a porous, electrically insulating membrane(called a separator), immersed into a liquid (called an electrolyte)enabling lithium ions transport.

In most systems, the electrodes are composed of an electrochemicallyactive material—meaning that it is able to chemically react with lithiumions to store and release them reversibly in a controlled manner—mixedif necessary with an electrically conductive additive (such as carbon)and a polymeric binder. A slurry of these components is coated as a thinfilm on a current collector (typically a thin foil of copper oraluminium), thus forming the electrode upon drying.

In the known Li ion battery technology, the poor rate capability ofgraphite anodes upon battery charging is a serious impediment to itsapplication in high-power electronics, automotive and industry. Among awide range of potential alternatives proposed recently, Si, Si alloysand lithium titanate (LTO), and niobium oxide-based materials are themain contenders to replace graphite as the active material of choice forhigh power applications.

Battery charge rate is usually expressed as a “C-rate”. 1 C charge ratemeans a charge current such that the battery is fully charged in 1 h, 10C charge means that the battery is fully charged in 1/10th of an hour (6minutes).

Batteries relying on a graphitic anode are fundamentally limited interms of charging rate. Under nominal conditions, lithium ions areinserted into the anode active material upon charging. When chargingrate increases, typical graphite voltage profiles are such that there isa high risk that overpotentials lead to the potential of the anode tobecome <0 V vs. Li/Li+, which leads to a phenomenon called lithiumdendrite electroplating, whereby lithium ions instead deposit at thesurface of the graphite electrode as lithium metal. This leads toirreversible loss of active lithium and hence rapid capacity fade of thecell. In some cases these dendritic deposits can grow to such largesizes that they pierce the battery separator and lead to a short-circuitof the cell. This can trigger a catastrophic failure of the cell leadingto a fire or an explosion. Accordingly, the fastest-charging batterieshaving graphitic anodes are limited to charging rates of 5-7 C, butoften less. Despite this, graphitic anodes accounted for >90% of theLi-ion battery market in 2018.

Si and Si alloys offer large specific capacities but suffer from poorlifetime when charged and discharged at high rates, as well as poorcapacity retention when increasing rates from low rates (e.g. 0.5 C) tohigher rates (e.g. 5 C). This is due to inhomogeneous lithiation of theactive material particles upon charging due to limited diffusion ratesof lithium ions within the particles. The core of the active materialparticles (typically 1-20 μm spheres) may not have time to be lithiatedupon fast charge as lithium ions need to diffuse to it from the particlesurface, hence poor capacity retention when increasing charge rate.Moreover, Si and Si alloys active materials physically expand up to 400%by volume upon lithiation.

Inhomogeneous particle lithiation therefore leads to internal mechanicalstresses within the particles, which can lead to their break up andelectrode pulverisation, hence the poor cycle life of these electrodesupon fast charging.

Lithium titanate (LTO) anodes do not suffer from dendrite electroplatingat high charging rate thanks to their high potential (1.6 V vs. Li/Li+),and have excellent cycle life as they do not suffer from volumeexpansion upon lithiation. LTO cells are typically regarded as highsafety cells for these two reasons. However LTO is a relatively poorelectronic and ionic conductor, which leads to limited capacityretention at high rate, unless the material is nanosized to increasespecific surface area, and carbon-coated to increase electronicconductivity. This particle-level material engineering increasesmaterial particle cost, and decreases the tapped-density of the activematerial LTO powders. This is significant because it leads to lowdensity electrodes and a higher fraction of electrochemically inactivematerial (e.g. binder, carbon additive).

A key measure of anode performance is its volumetric capacity (mAh/cm³),that is, the amount of electric charges (that is lithium ions) that canbe stored per unit volume of the anode. This is an important factor todetermine the overall battery energy density on a volumetric basis(Wh/L). Volumetric capacity can be approximated as the product ofelectrode density, active material specific capacity, and fraction ofactive material in the electrode. LTO anodes typically have relativelylow specific capacities (c. 170 mAh/g, to be compared with c. 330 mAh/gfor graphite), which, combined with their low electrode densities(typically 1.9 g/cm³) and low active material fractions (<87%) discussedabove, lead to very low volumetric capacities (<300 mAh/cm³) andtherefore low battery energy density and high $/kWh cost. As a result,LTO batteries/cells are generally limited to specific nicheapplications, despite their long cycle life, fast- charging capability,and high safety.

Mixed niobium oxides (MNO) were first identified as potential batterymaterials in the academic literature in the 1980's [Cava et al. (1983);Cava et al. (1984)], but generated limited interest at the time becauseof the lack of commercially available cathodes to match their rateperformance.

Interest in MNO anodes was revived in the early 2010's, with thedemonstration of a practical cell combining a TiNb₂O₇ anode and acommercially-available LNMO cathode showing promising performance interms of rate capability, cycle life, and energy density [Goodenough andPark (2013)]. Selected MNO anodes such as TiNb₂O₇ offer characteristicsthat are similar to LTO in terms of high operating potential vs. Li/Li+(1.6V) and low volume expansion (<5%) leading to safe fast-charge andlong cycle life (>10,000 cycles). A key advantage of MNO anodes is thatspecific capacities significantly higher than LTO can be achieved (e.g.c. 300 mAh/g for TiNb₂O₇), which improves cell energy density. However,electronic conductivities are typically too low in MNO materials such asTiNb₂O₇ to sustain fast charge rates without particle engineering andcarbon coatings, which is a limitation similar to that of LTO.

On the other hand, it was recently shown that other MNOs such as Nb₂O₅,also studied in the 1980's for batteries, which typically have aso-called “Wadsley-Roth” or “bronze” crystal structure, can offerextremely fast lithium ion diffusion rates 10⁻¹⁴−10⁻¹⁰ cm²s⁻¹ (LTOtypically 10⁻¹⁷ cm² S⁻¹) [Griffith et al. (2016)]. This can potentiallyimprove on electrode density (i.e. >2.5 g/cm³), and thus in volumetriccapacity (>600 mAh/cm³) and cell energy density. However, severalchallenges limit the commercial deployment of these materials, such aslow electronic conductivity, lifetime issues as a “micromaterial”(crystals on the order of 1-10 μm in size), and “sloping” lithiationvoltage profiles. A lithiation voltage profile refers to the shape ofthe anode potential vs. amount of lithium inserted in the anode. LTO and

TiNb₂O₇ have a “flat” voltage profile whereas materials such as Nb₂O₅typically have a “sloping” voltage profile. Voltage profiles that aretoo sloping lead to large voltage windows which make full cell balancingchallenging in a commercial cell.

TiNb₂O₇ in particular has further limitations to its application in highpower battery technologies. It has a limited Li-ion diffusion rate(8.0×10⁻¹⁶ cm² s⁻¹) as compared to other MNOs (eg. Nb₁₂MoO₃₃=4.0×10⁻¹⁴cm² s⁻¹) [Zhu 2019], which limits its performance at high power. Inparticular this limits the utilisation of the pseudocapacitive chargestorage mechanism, a key benefit to the use of MNOs for high power [Yang2017].

U.S. Pat. No. 9,515,319B2 discusses TiNb₂O₇ and contemplates but doesnot exemplify modifications of this material. However the feedstocks andprocesses used in this disclosure are expensive (furnace treatments to1500° C. up to 50 h), and the materials produced show low initialCoulombic efficiency (84.7%, 86.5%). US2015/0270543A1 and KR20150131800Adisclose modifying TiNb₂O₇.

US2019/0288283A1 discloses a lithium niobium composite oxide where as anessential feature some of the niobium must be replaced by at least oneelement selected from Fe, Mg, Al, Cu, Mn, Co, Ni, Zn, Sn, Ti, Ta, V, andMo.

The present invention has been devised in light of the aboveconsiderations.

SUMMARY OF THE INVENTION

The present inventors have realised that, despite the apparentchallenges presented by the prior art, it is possible to provide anactive electrode material which overcomes some or all of the problemspresented by prior art materials discussed above.

Accordingly, in a first aspect, the present invention provides an activeelectrode material expressed by the general formula[M1]_(x)[M2]_((1-x))[Nb]_(y)[O]_(z), wherein:

-   -   M1 and M2 are different;    -   M1 represents one or more of Ti, Mg, V, Cr, W, Zr, Mo, Cu, Fe,        Ga, Ge, Ca, K, Ni, Co, Al, Sn, Mn, Ce, Te, Se, Si, Sb, Y, La,        Hf, Ta, Re, Zn, In, or Cd;    -   M2 represents one or more of Mg, V, Cr, W, Zr, Mo, Cu, Ga, Ge,        Ca, K, Ni, Co, Al, Sn, Mn, Ce, Sb, Y, La, Hf, Ta, Zn, In, or Cd;        and wherein    -   x satisfies 0<x<0.5;    -   y satisfies 0.5≤y≤49    -   z satisfies 4≤z≤124.

A material in which M1 and M2 are different can also be referred to as amixed cation active materials, or a complex oxide active material. Theseterms are used interchangeably in the present disclosure to refer to amaterial of the general formula as set out above. Such materials mayoffer improved electrochemical properties in comparison to non-mixedcation active materials (e.g. materials having the general formula[M]_(x)[Nb]_(y)[O]_(z), where M represents a single ion.

In particular, as shown by the examples, the inventors have found thatby substituting the non-Nb cation to form a mixed cation structure theentropy can increase in the crystal structure, reducing potential energybarriers to Li ion diffusion through minor defect introduction.Modification by creating mixed cation structures that retain the sameoverall oxidation state as the unmodified crystal structure demonstratethe potential improvements by altering ionic radii, which can causeminor changes in crystal parameters and Li-ion cavities that can improveelectrochemical properties. For example, by substituting with a cationof larger ionic radius, the unit cell can be expanded versus theunmodified structure, which can result in higher Li ion diffusion rates.Modification by creating mixed cation structures that result inincreased oxidation state demonstrate similar potential advantages withaltered ionic radii, compounded by introduction of additional electronholes in the structure to aid in electrical conductivity. Modificationby creating mixed cation structures that result in decreased oxidationstate demonstrate similar potential advantages with altered ionic radii,compounded by introduction of oxygen vacancies and additional electronsin the structure to aid in electrical conductivity. Modification byinducing oxygen deficiency from high temperature treatment in inert orreducing conditions provides a reduced structure of much improvedelectrical conductivity. Combination of mixed cation structures andinduced oxygen deficiency allows multiple beneficial effects.

As set out above, M1 represents one or more of Ti, Mg, V, Cr, W, Zr, Mo,Cu, Fe, Ga, Ge, Ca, K, Ni, Co, Al, Sn, Mn, Ce, Te, Se, Si, Sb, Y, La,Hf, Ta, Re, Zn, In, or Cd. M2 represents one or more of Mg, V, Cr, W,Zr, Mo, Cu, Ga, Ge, Ca, K, Ni, Co, Al, Sn, Mn, Ce, Sb, Y, La, Hf, Ta,Zn, In, or Cd. By ‘represents one or more of’, it is intended thateither M1 or M2 may each represent two or more elements from theirrespective lists. An example of such a material isTi_(0.05)W_(0.25)Mo_(0.70)Nb₁₂O₃₃. Here, M1 represents Ti_(x′)W_(x″)(where x′+x″=x), M2 represents Mo, x=0.3, y=12, z=33. Another example ofsuch a material is T_(0.05)Zr_(0.05)W_(0.25)Mo0.65Nb₁₂O₃₃. Here, M1represents Ti_(x′)Zr_(x″)W_(x′″) (where x′+x″+x′″=x), M2 represents Mo,x=0.35, y=12, z=33.

Optionally M1 represents one or more of K, Mg, Ca, Y, Ti, Zr, Hf, V, Ta,Cr, Mo, W, Mn, Fe, Co, Ni, Cu, Zn, Al, Ga, Si, Ge, Sn, Sb. M1 mayrepresent one or more of Ti, Mg, V, Cr, W, Zr, Mo, Cu, Ga, Ge, K, Ni,Al, Hf, Ta, or Zn. Preferably, M1 represents one or more of Ti, Mg, V,Cr, W, Zr, Mo, Ga, Ge, Al, or Zn.

M2 does not represent Ti. In other words, preferably, Ti is not themajor non-Nb cation in the active electrode material. Where M1represents Ti alone, preferably x is 0.05 or less. Where M1 representsone or more cations including Ti, preferably the amount of Ti relativeto the total amount of non-Nb cations is 0.05:1 or less.

Optionally M2 is selected from one or more of Mo, W, V, Zr, Al, Zn, Ga,Ge, Ta, Cr, Cu, K, Mg, Ni, or Hf, M2 may be selected from one or more ofMo, W, V, Zr, Al, Zn, Ga, or Ge. Preferably, M2 is selected from one ormore of Mo, W, V, or Zr. The present inventors have found that when M2is selected from one of these elements, the active electrode materialmay have improved electrochemical properties. M2 may consist of a singleelement.

As x satisfies 0<x<0.5, M2 is the major non-Nb cation in the activeelectrode material. Preferably x satisfies 0.01≤x≤0.4, more preferably xsatisfies 0.05≤x≤0.25, for example, x may be about 0.05.

The precise values of y and z within the ranges defined may be selectedto provide a charge balanced, or substantially charge balanced, crystalstructure. Additionally or alternatively, the precise values of y and zwithin the ranges defined may be selected to provide a thermodynamicallystable, or thermodynamically metastable, crystal structure.

In some cases, z may be defined in the format z=(z′−z′a), where a is anon-integer value less than 1, for example where a satisfies 0≤α≤0.05. αmay be greater than 0, i.e. a may satisfy 0≤α≤0.05. When a is greaterthan 0, the active electrode material is an oxygen deficient material,i.e. the material has oxygen vacancies. Such a material would not haveprecise charge balance, but is considered to be “substantially chargebalanced” as indicated above. Alternatively, α may equal 0, in whichcase the material is not an oxygen deficient material.

When α is 0.05, the number of oxygen vacancies is equivalent to 5% ofthe total oxygen in the crystal structure. In some embodiments, a may begreater than 0.001 (0.1% oxygen vacancies), greater than 0.002 (0.2%oxygen vacancies), greater than 0.005 (0.5% oxygen vacancies), orgreater than 0.01 (1% oxygen vacancies). In some embodiments, α may beless than 0.04 (4% oxygen vacancies), less than 0.03 (3% oxygenvacancies), less than 0.02 (2% oxygen vacancies), or less than 0.1 (1%oxygen vacancies). For example, a may satisfy 0.001≤α≤0.05. When thematerial is oxygen deficient, the electrochemical properties of thematerial may be improved, for example, resistance measurements may showimproved conductivity in comparison to equivalent non-oxygen deficientmaterials. As will be understood, the percentage values expressed hereare in atomic percent.

The oxygen deficiency (e.g. expressed as the percentage of oxygenvacancies) in a material can be measured by e.g. thermogravimetricanalysis (TGA) in an oxygen-rich atmosphere, by measurement of how themass of the sample changes over time due to re-inclusion of oxygen inthe oxygen vacancies.

Alternatively or additionally, the oxygen deficiency can bequalitatively measured by assessing the colour of a material relative toa non-oxygen deficient sample of the same material. For example,non-oxygen deficient MoNb₁₂O₃₃ has a white, off-white, or yellow colour.Oxygen-deficient MoNb₁₂O_(<33) has a purple colour. On production of anoxygen deficient crystal of MoNb₁₂O_(<33) a colour change fromwhite/off-white/yellow to purple can be observed.

M1 may have an equal or lower oxidation state than M2. Preferably, M1has a lower oxidation state than M2. When more than one element ispresent as M1 and/or M2 it will be understood that the oxidation staterefers to M1 and/or M2 as a whole. For example, if 25 at% of M1 is Tiand 75 at% of M1 is W the oxidation state of M1 is 0.25×4 (thecontribution from Ti)+0.75×6 (the contribution from W). Advantageously,when M1 has a lower oxidation state than M2 this is compensated for bythe formation of oxygen vacancies, i.e. forming an oxygen deficientactive electrode material. The presence of oxygen vacancies is believedto improve the conductivity of the active electrode material and toprovide further benefits, as evidenced by the examples. Optionally, M1comprises at least one cation with a 4+ oxidation state and M2 comprisesat least one cation with a 6+ oxidation state. Optionally, M1 has anoxidation state of 4+ and M2 has an oxidation state of 6+. M1 preferablyhas a different ionic radius than M2, most preferably a larger ionicradius. This gives rise to changing unit cell size and local distortionsin crystal structure. This is believed to improve electrochemicalproperties such as specific capacity and Coulombic efficiency throughaltering the Li ion site availability by varying cavity size andreduction of energy barriers to reversible lithiation. The ionic radiimay be the Shannon ionic radii (available at R. D. Shannon, Acta Cryst.,A32, 1976, 751-76) at the coordination and valency that the ion would beexpected to adopt in the crystal structure of the active electrodematerial.

The active electrode material may be material selected from the groupconsisting of:

-   (i) M1_(x)Mo_((1-x))Nb₁₂O_((33-33 α))-   M1_(x)W_((1-x))Nb₁₂O_((33-33 α))-   M1_(x)V_((1-x))Nb₉O_((25-25 α))-   M1_(x)Zr_((1-x))Nb₂₄O_((62-62 α))-   M1_(x)W_((1-x))Nb_(0.57)O_((4.43-4.43 α))-   M1_(x)W_((1-x))Nb_(0.89)O_((5.22-5.22 α))-   M1_(x)Zn_((1-x))Nb₁₇O_((43.5-43.5 α))-   M1_(x)Cu_((1-x))Nb₁₇O_((43.5-43.5 α))-   M1_(x)Al_((1-x))Nb₁₁O_((29-29 α))-   M1_(x)Ga_((1-x))Nb₁₁O_((29-29 α))-   M1_(x)Ge_((1-x))Nb₁₈O_((47-47 α))-   M1_(x)W_((1-x))Nb_(1.125)O_((5.81-5.81 α))-   M1_(x)W_((1-x))Nb_(3.2)O_((11-11 α))-   M1_(x)Al_((1-x))Nb₄₉O_((124-124 α))-   M1_(x)Ga_((1-x))Nb₄₉O_((124-124 α)); or-   (ii) M1_(x)Mo_((1-x))Nb₁₂O_((33-33 α))-   M1_(x)W_((1-x))Nb₁₂O_((33-33 α))-   M1_(x)V_((1-x))Nb₉O_((25-25 α))-   M1_(x)Zr_((1-x))Nb₂₄O_((62-62 α))-   M1_(x)W_((1-x))Nb_(0.57)O_((4.43-4.43 α))-   M1_(x)W_((1-x))Nb_(0.89)O_((5.22-5.22 α))-   M1_(x)Zn_((1-x))Nb₁₇O_((43.5-43.5 α))-   M1_(x)Al_((1-x))Nb₁₁O_((29-29 α))-   M1_(x)Ge_((1-x))Nb₁₈O_((47-47 α)); or preferably-   (iii) M1_(x)Mo_((1-x))Nb₁₂O_((33-33 α))-   M1_(x)W_((1-x))Nb₁₂O_((33-33 α))-   M1_(x)V_((1-x))Nb₉O_((25-25 α))-   M1_(x)Zr_((1-x))Nb₂₄O_((62-62 α))-   M1_(x)W_((1-x))Nb_(0.57)O_((4.43-4.43 α))-   M1_(x)W_((1-x))Nb_(0.89)O_((5.22-5.22 α))

where M1 represents one or more of Ti, Mg, V, Cr, W, Zr, Mo, Cu, Fe, Ga,Ge, Ca, K, Ni, Co, Al, Sn, Mn, Ce, Te, Se, Si, Sb, Y, La, Hf, Ta, Re,Zn, In, or Cd; and wherein x satisfies 0<x<0.5; and a satisfies0≤α≤0.05.

In a particularly preferred aspect, the active electrode material isM1_(x)Mo_((1-x))Nb₁₂O_((33-33 α)). In another particularly preferredaspect, the active electrode material isM1_(x)W_((1-x))Nb_(0.57)O_((4.43-4.43 α)). In another particularlypreferred aspect, the active electrode material isM1_(x)Zn_((1-x))Nb₁₇O_((43.5-43.5 α)). In another particularly preferredaspect, the active electrode material isM1_(x)Al_((1-x))Nb₁₁O_((29-29 α)). The examples show that thesematerials have particularly advantageous properties for use as activeelectrode materials.

The materials above in groups (i), (ii), and (iii) and in theparticularly preferred aspects represent specific non-mixed cationactive materials (i.e. when x=0) which have been modified into mixedcation active materials by the substitution of less than half of M2 by adifferent element M1. Optionally, in these materials, M2 may also besubstituted by Nb on the non-Nb site of the crystal structure. That is,M1 can represent one or more of Ti, Mg, V, Cr, W, Zr, Mo, Cu, Fe, Ga,Ge, Ca, K, Ni, Co, Al, Sn, Mn, Ce, Te, Se, Si, Sb, Y, La, Hf, Ta, Re,Zn, In, Cd, or Nb. M1 can also represent further list of elementsrecited above and in the claims.

In a second aspect, the present invention provides an active electrodematerial expressed by the general formula [M][Nb]_(y)[O]_(z); whereinthe active electrode material is oxygen deficient; wherein M consists ofone of Mg, Cr, W, Mo, Cu, Ga, Ge, Ca, K, Ni, Co, Al, Sn, Mn, Ce, Sb, Y,La, Hf, Ta, Zn, In, or Cd; y satisfies 0.5≤y≤49; and z satisfies4≤z≤124.

Materials according to the second aspect of the invention are oxygendeficient analogues of known ‘base’ materials such as MoNb₁₂O₃₃,WNb₁₂O₃₁, W₇Nb₈O₃₁, and W₉Nb₈O₄₇. z may be defined as z=(z′−z′α) whereina satisfies 0≤α≤0.05. The comments set out above in relation tomaterials according to the first aspect specifying possible ranges for αwhen z is defined as z=(z′−z′α) also apply here to materials of thesecond aspect of the invention. For example, α may satisfy 0.001≤α≤0.05.It has been found that oxygen deficient materials according to thesecond aspect have improved properties for use as active electrodematerials compared to the stoichiometric ‘base’ materials. For example,the materials according to the second aspect have improved electricalconductivity.

M may consist of one of Mo, W, Al, Zn, Ga, Ge, Ta, Cr, Cu, K, Mg, Ni, orHf; or M may consist of one of Mo, W, Al, Zn, Ga, or Ge; or preferably Mmay consist of one of Mo, W, Al, or Zn.

The active electrode material of the second aspect may be expressed bythe general formula [M]_(x)[Nb]_(y)[O]_((Z′−z′α)), selected from thegroup consisting of:

-   MoNb₁₂O_((33-33 α))-   WNb₁₂O_((33-33 α))-   W₇Nb₄O_((31-31 α))-   W₉Nb₈O_((47-47 α))-   Zn₂Nb₃₄O_((87-87 α))-   Cu₂Nb₃₄O_((87-87 α))-   AlNb₁₁O_((29-29 α))-   GaNb₁₁O_((29-29 α))-   GeNb₁₈O_((47-47 α))-   W₁₆Nb₁₈O_((93-93 α))-   W₅Nb₁₆O_((55-55 α))-   AlNb₄₉O_((124-124 α))-   GaNb₄₉O_((124-124 α))-   wherein α satisfies 0<α≤0.05.

These represent specific stoichiometric mixed niobium oxides which havebeen modified into oxygen deficient mixed niobium oxides.

The active electrode material of the second aspect may be expressed bythe general formula [M]_(x)[Nb]_(y)[O]_((Z′−z′α)), selected from thegroup consisting of:

-   MoNb₁₂O_((33-33 α))-   WNb₁₂O_((33-33 α))-   W₇Nb₄O_((31-31 α))-   W₉Nb₈O_((47-47 α))-   Zn₂Nb₃₄O_((87-87 α))-   AlNb₁₁O_((29-29 α))-   GaNb₄₉O_((124-124 α))-   wherein α satisfies 0<α≤0.05.

The active electrode material of the second aspect may be expressed bythe general formula [M]_(x)[Nb]_(y)[O]_((Z′−z′α)), selected from thegroup consisting of:

-   MoNb₁₂O_((33-33 α))-   WNb₁₂O_((33-33 α))-   W₇Nb₄O_((31-31 α))-   W₉Nb₈O_((47-47 α))-   wherein a satisfies 0<α≤0.05.

Optionally, M is W. That is, the active electrode material may beexpressed by the general formula [W][Nb]_(y)[O]_(z). For example, theactive electrode material may be selected from WNb₁₂O_((33-33 α)),W₇Nb₄O_((31-31 α)), W₉Nb₈O_((47-47 α)), W₁₆Nb₁₈O_((93-93 α)), andW₅Nb₁₆O_((55-55 α)). The examples demonstrate that inducing oxygendeficiency in a number of different tungsten niobium oxides leads toimproved properties, e.g. improved electrical conductivity, compared tothe stoichiometric base oxides.

In a particularly preferred aspect, the active electrode material isMoNb₁₂O_((33-33 α)). In another particularly preferred aspect, theactive electrode material is WNb₁₂₀(33-33 a). In another particularlypreferred aspect, the active electrode material is W₆Nb₁₆O_((66-66 α)).In another particularly preferred aspect, the active electrode materialis W₇Nb₄O_((31-31 α)). In another particularly preferred aspect, theactive electrode material is Zn₂Nb₃₄O_((87-87 α)). In anotherparticularly preferred aspect, the active electrode material isAlNb₁₁O_((29-29 α)). The examples show that these materials haveparticularly advantageous properties for use as active electrodematerials.

The present inventors have found that by modifying materials such asMoNb₁₂O₃₃, WNb₁₂O₃₃, ZrNb₂₄O₆₂, VNb₉O₂₅, W₇Nb₄O₃₁, and W₉Nb₈O₄₇ byeither incorporating multiple non-Nb cations to form mixed cation activematerials/complex oxide active materials (as per the first aspect of theinvention), and/or by creating an oxygen deficiency (as per the secondaspect of the invention), it is possible to create active electrodematerials having improved electrochemical properties, and in particularimproved electrochemical properties when used as an anode material.

The specific capacity/reversible delithiation capacity of activeelectrode materials according to the invention may be 200 mAh/g or more,225 mAh/g or more, 250 mAh/g or more, up to about 300 mAh/g or more.Here, specific capacity is defined as that measured in the 2nd cycle ofa half cell galvanostatic cycling test at a rate of 0.05C with a voltagewindow of 1.1-3.0V vs Li/Li+. It may be advantageous to providematerials having a high specific capacity, as this can provide improvedperformance in an electrochemical device comprising the active electrodematerial.

Furthermore, active electrode materials according to the invention mayhave an appropriate voltage profile as measured in the 2^(nd) cycle of ahalf cell galvanostatic cycling test at a rate of 0.05C with a voltagewindow of 1.1-3.0V vs Li/Li+. That is, the capacity of the material maybe between 2.0 V and 1.1 V upon lithiation >180 mAh/g, and the capacityof the material may be between 1.1 V and 2.0V upon delithiation >180mAh/g.

When formulated or coated as an electrode (optionally with conductivecarbon additive and binder materials), the bulk resistivity of activeelectrode materials according to the invention, may be 5 kΩ.cm or less,more preferably 2 kΩ.cm or less. Bulk resistivity can be a useful proxymeasurement of the electronic conductivity of such materials. It may beadvantageous to provide materials having a suitably low bulkresistivity, as this can provide improved performance in anelectrochemical device comprising the active electrode material.

The direct current internal resistance (DCIR) and resultant areaspecific impedance (ASI) of the active electrode material when measuredin a Li-ion half coin cell with the described electrode may be 90 Ω orless (for DCIR) and 170 Ω.cm² or less (for ASI). It may be advantageousto provide materials having a suitably low DCIR and/or ASI, as this canprovide improved performance in an electrochemical device comprising theactive electrode material. However, further improvements in DCIR/ASIvalues may be seen for e.g. carbon coated active electrode materials, orwhere the active electrode material is incorporated in a commercialpower cell with a cathode, with an electrode which has been calendaredand prepared in a typical known manner. When measured in such anarrangement in a coin cell, the inventors theorise that the ASI may beas low as e.g. 26 Ω.cm² or less.

Active electrode materials according to the invention may have a lithiumdiffusion rate of greater than 10⁻¹⁴cm²s⁻¹. It may be advantageous toprovide materials having a suitably high lithium diffusion rate, as thiscan provide improved performance in an electrochemical device comprisingthe active electrode material.

Active electrode materials according to the invention may have anelectrode density of 2.5 g/cm³ or more after calendaring. For example,electrode densities of up to 3.0 g/cm³ or more after calendaring havebeen achieved. It may be advantageous to provide materials having suchan electrode density, as this can provide improved performance in anelectrochemical device comprising the active electrode material.Specifically, when the electrode density is high, high volumetriccapacities can be achieved, as gravimetric capacity x electrodedensity×active material fraction=volumetric capacity.

The initial coulombic efficiency of active electrode materials accordingto the invention may be greater than 88%, more preferably greater than90%. In some cases the initial coulombic efficiency of the activeelectrode material may be as high as 92% or more, 93% or more or 94% ormore. It may be advantageous to provide materials having a suitably highinitial coulombic efficiency, as this can provide improved performancein an electrochemical device comprising the active electrode material.Initial coulombic efficiency may be measured as the difference in thelithiation and de-lithiation capacity on the 1^(st) charge/dischargecycle at C/10 in a half-cell.

Further optional features of the first and second aspects of theinvention are set out below.

The crystal structure of the active electrode material of the firstaspect, as determined by X-ray diffraction, may correspond to thecrystal structure of the unmodified form of the active electrodematerial, wherein the unmodified form is expressed by the formula[M2][NID]_(y)[O]_(z) wherein M2 consists of a single element and whereinthe unmodified form is not oxygen deficient, wherein the unmodified formis selected from one or more of: M2^(I)Nb₅O₁₃, M2^(I)Nb_(10.8)O₃₀,M2^(II) ₂Nb₃₄O₈₇, M2^(III)Nb₁₁O₂₉, M2^(III)Nb₄₉O₁₂₄ (M2^(III)_(0.5)Nb_(24.5)O₆₂), M2^(IV)Nb₂₄O₆₂, M2^(IV)Nb₂O₇, M2^(IV) ₂Nb₁₀O₂₉,M2^(IV)Nb₁₄O₃₉, M2^(IV)Nb₁₄O₃₇, M2^(IV)Nb₆O₁₇, M2^(IV)Nb₁₈O₄₇,M2^(V)Nb₉O₂₅, M2^(V) ₄Nb₁₈O₅₅, M2^(V) ₃Nb₁₇O₅₀, M2^(VI), Nb₁₂O₃₃,M2^(VI) ₄Nb₂₆O₇₇, M2^(VI) ₉Nb₁₄O₄₄, M2^(VI) ₅Nb₁₆O₅₅, M2^(VI) ₈Nb₁₈O₆₉,M2^(VI) ₁₆Nb₁₈O₉₃, M2^(VI) ₂₀Nb₂₂O₁₁₅, M2^(VI) ₉Nb₈O₄₇, M2^(VI)₈₂Nb₅₄O₃₈₁, M2^(VI) ₃₁Nb₂₀O₁₄₃, M2^(VI) ₇Nb₄O₃₁, M2^(VI) ₁₅Nb₂O₅₀,M2^(VI) ₃Nb₂O₁₄, and M2^(VI) ₁₁Nb₂O₆₃, wherein the numerals I, II, III,IV, V, and VI represent the oxidation state of M2. In this way, it canbe confirmed that the unmodified form has been modified withoutsignificantly affecting the crystal structure.

The crystal structure of the active electrode material of the secondaspect, as determined by X-ray diffraction, may correspond to thecrystal structure of the unmodified form of the active electrodematerial, wherein the unmodified form is expressed by the generalformula [M][Nb]_(y)[O]_(z) wherein the unmodified form is not oxygendeficient, wherein the unmodified form is selected from M2^(I)Nb₅O₁₃,M2^(I) ₆Nb_(10.8)O₃₀, M2^(II)Nb₂O₆, M2^(II) ₂Nb₃₄O₈₇, M2^(III)Nb₁₁O₂₉,M2^(III)Nb₄₉O₁₂₄, M2^(IV)Nb₂₄O₆₂, M2^(IV)Nb₂O₇, M2^(IV) ₂Nb₂Nb₁₀O₂₉,M2^(IV) ₂Nb₁₄O₃₉, M2^(IV)Nb₆O₁₇, M2^(IV)Nb₆O₁₇, M2^(IV)Nb₁₈O₄₇,M2^(V)Nb₉O₂₅, M2^(V) ₄Nb₁₈O₅₅, M2^(V) ₃Nb₁₇O₅₀, M2^(VI)Nb₁₂O₃₃, M2^(VI)₄Nb₂₆O₇₇, M2^(VI) ₃Nb₁₄O₄₄, M2^(VI) ₅Nb₁₆O₅₅, M2^(VI) ₈Nb₁₈O₆₉,M2^(IV)Nb₂O₈, M2^(VI) ₁₆Nb₂O₈, M2^(VI) ₁₆Nb₁₈O₉₃, M2^(VI) ₂₀Nb₂₂O₁₁₅,M2^(VI) ₉Nb₈O₄₇, M2^(VI) ₈₂Nb₅₄O₃₈₁, M2^(VI) ₃₁Nb₂₀O₁₄₃, M2^(VI)₇Nb₄O₃₁, M2^(VI) ₁₅Nb₂O₅₀, M2^(VI) ₃₁Nb₂₀O₅₀, M2^(VI) ₃₁Nb₂₀, O₁₄₃,M2^(VI) ₇Nb₂O₁₄, and M2^(VI) ₁₁Nb₁₂O₆₃, wherein the numerals I, II, III,IV, V, and VI represent the oxidation state of M. In this way, it can beconfirmed that the unmodified form has been modified withoutsignificantly affecting the crystal structure.

The crystal structure of the active electrode material, as determined byX-ray diffraction analysis, may correspond to the crystal structure ofone or more of:

-   (i) MoNb₁₂O₃₃-   WNb₁₂O₃₃-   VNb₉O₂₅-   ZrNb₂₄O₆₂-   W₇Nb₄O₃₁-   W₉Nb₈O₄₇-   Zn₂Nb₃₄O₈₇-   Cu₂Nb₃₄O₈₇-   AlNbNb₁₁O₂₉-   GaNb₁₁O₂₉-   GeNb₁₈O₄₇-   W₁₆Nb₁₈O₉₃-   W₅Nb₁₆O₅₅-   AlNb₄₉O₁₂₄-   GaNb₄₉O₁₂₄; or-   (ii) MoNb₁₂O₃₃-   WNb₁₂O₃₃-   VNb₉O₂₅-   ZrNb₂₄O₆₂-   W₄Nb₇O₃₁-   W₉Nb₈O₄₇-   Zn₂Nb₃₄O₈₇-   AlNb₁₁O₂₉-   GeNb₁₈O₄₇; or preferably-   (iii) MoNb₁₂O₃₃-   WNb₁₂O₃₃-   ZrNb₂₄O₆₂-   VNb₉O₂₅-   W₇Nb₄O₃₁-   W₉Nb₈O₄₇.

Here the term ‘corresponds’ is intended to reflect that peaks identifiedby X-ray diffraction analysis of the active electrode material may beshifted by no more than 0.5 degrees (preferably shifted by no more than0.2 degrees, more preferably shifted by no more than 0.1 degrees) fromcorresponding peaks in an X-ray diffraction analysis of one or more ofthe reference crystal structure (e.g. MoNb_(12033,)WNID12033, ZrNb₂₄O₆₂,VNb₉O₂₅, W₇Nb₄O₃₁, and/or W₉Nb₈O₄₇). Preferably the crystal structure ofthe active electrode material does not correspond to the crystalstructure of TiNb₂O₇, for example, preferably the measured XRDdiffraction pattern of the active electrode material does not correspondto the JCPDS crystallography database entry database 00-039-1407, forTiNb₂O₇. Optionally, the crystal structure of the active electrodematerial does not correspond to the crystal structure of Ti2Nb₁₀₀₂₉.Optionally, the crystal structure of the active electrode material doesnot correspond to the crystal structure of M^(III)Nb₁₁O₂₉ for exampleFeNb₁₁O₂₉, GaNb₁₁O₂₉, CrNb₁₁O₂₉, and AlNb₁₁O₂₉.

At least some of the active electrode material may have a Wadsley-Rothcrystal structure and/or a tetragonal tungsten bronze (TTB) crystalstructure. Preferably, the majority of the active electrode material hasa Wadsley-Roth crystal structure and/or a tetragonal tungsten bronze(TTB) crystal structure, for example, at least 50%, at least 60%, atleast 70%, at least 80% or at least 90% of the active electrode materialby volume may have a Wadsley-Roth crystal structure and/or a tetragonaltungsten bronze (TTB) crystal structure. In preferred embodiments,substantially all of the active electrode material may have aWadsley-Roth crystal structure and/or a tetragonal tungsten bronze (TTB)crystal structure. When the material has such a crystal structure it mayhave improved electrochemical properties.

The crystal formula of a charge balanced and thermodynamically stableWadsley-Roth crystal structure obeys the following formula:

(M₁, M₂, M₃, . . . )_(3mnp−(m+n)p+4)   (1)

In this formula, O is oxygen (the anion) and M (the cation) is anycombination of elements selected from Ti, Mg, V, Cr, W, Zr, Nb, Mo, Cu,Fe, Ga, Ge, Ca, K, Ni, Co, Al, Sn, Mn, Ce, Te, Se, Si, Sb, Y, La, Hf,Ta, Re, Zn, In, or Cd. In materials according to the invention, at leastone of (M1, M2, M3 . . . ) comprises Nb.

Formula (1) is based on crystal topography: m and n are the dimensionsof the formed edge sharing superstructure blocks, ranging from 3-5(integers). At the corner, blocks are connected into infinite ribbons(p=∞) only by edge-sharing, into pairs (p=2) by partly edge-sharing andpartly tetrahedra or into isolated blocks only by tetrahedra (p=1). Whenp is infinity the formula becomes:

(M₁, M₂, M₃, . . . )_(mn)O_(3mn−(m+n))   (2)

Together, formula (1) and (2) define the full composition samples forWadsley-Roth crystal structures. Preferably the total crystalcomposition should also be charge neutral and thermodynamicallyfavourable.

More information can be found in work by Griffith et al. (2017).

Reference to tetragonal tungsten bronze (TTB) crystal structures (orsimply ‘bronze’ structures) in the present disclose refers to tetragonaltungsten bronze (TTB) structures with partially filled tunnels. Asdescribed in Montemayor 1998, such phases consist in a framework of NbO₆octahedra sharing corners linked in such a way that three, four and fivesided tunnels are formed. A number of 5-sided tunnels are filled with W,Nb, O, or a suitable metal cation to form the structure.

The active electrode material may further comprise Li and/or Na. Inother words, the active electrode material may be a lithiated and/or asodiated active electrode material. The active electrode material of thefirst aspect may be expressed by the general formula[Li]_(λ)[M1]_(x)[M2]_((1-x))[Nb]_(y)[O]_(z) or[Na]_(λ)[M1]_(x)[M2]_((1−x))[Nb]_(y)[O]_(z). The active electrodematerial of the second aspect may be expressed by the general formula[Li]_(λ)[M][Nb]_(y)[O]_(z) or [Na]_(λ)[M][Nb]_(y)[O]_(z). x, y, and zsatisfy the ranges discussed above, and λ is selected to provide acharge balanced, or substantially charge balanced, crystal structure,and/or a thermodynamically stable, or thermodynamically metastable,crystal structure.

The active electrode material may have a BET surface area in the rangeof 0.1-100 m²/g, or 0.5-50 m²/g, or 1-20 m²/g. In general, a low BETsurface area is preferred in order to minimise the reaction of theactive electrode material with the electrolyte, e.g. minimising theformation of solid electrolyte interphase (SEI) layers during the firstcharge-discharge cycle of an electrode comprising the material. However,a

BET surface area which is too low results in unacceptably low chargingrate and capacity due to the inaccessibility of the bulk of the activeelectrode material to metal ions in the surrounding electrolyte. Theterm “BET surface area” refers to the surface area per unit masscalculated from a measurement of the physical adsorption of gasmolecules on a solid surface, using the Brunauer—Emmett—Teller theory.For example, BET surface areas can be determined in accordance with ISO9277:2010.

The active electrode material may comprise a plurality of primarycrystallites (sometimes referred to as microcrystals ormicrocrystallites). The average diameter of the primary crystallites maybe from 10 nm to 10 μm, preferably from 100 nm to 5 μm, although themost desirable diameter for the primary crystallites may depend on theirintended use. For example, where the active electrode material isintended for use in ultra-high power products, it may be advantageousfor the primary crystallite size to be low, e.g. 50 nm or less, or 30 nmor less. Where the active electrode material is intended for use indeveloping “high energy power cells”, it may be advantageous for thecrystallite size to be higher, e.g. 5 μm or more, or 7 μm or more.

Some or all of these primary crystallites may be agglomerated intosecondary particles. Alternatively, the primary crystallites may besubstantially non-agglomerated. Where some or all of these primarycrystallites are agglomerated into secondary particles, the averagediameter of the secondary particles (for example, the Dso diameter whenmeasured using solid state powder laser diffraction) is from 1 μm to 30μm, preferably from 2 μm to 15 μm, although the most desirable diameterfor the secondary particles may depend on their intended use. Forexample, where the active electrode material is intended for use inultra-high power products, it may be advantageous for the secondaryparticle size to be low, e.g. 4 μm or less, 2 μm or less, or 1.5 μm orless. Where the active electrode material is intended for use indeveloping “high energy power cells”, it may be advantageous for thesecondary particle size to be higher, e.g. 8 μm or more, 12 μm or more,or 15 μm or more. The secondary particles may be porous.

The average diameter of the primary crystallites and/or the secondaryparticles may be measured using any conventional known technique, forexample using SEM imaging to examine a sample of the material, selectinga number (n) of primary crystallites and/or secondary particles, andcalculating the average diameter as the mean diameter of the n primarycrystallites/secondary particles measured, e.g. where n equals 30.

An alternative method for measuring the secondary particle size is usingsolid state powder laser diffraction, for example using a Horiba laserdiffraction particle analyser for dry powder with air pressuremaintained at 0.3 MPa.

The active electrode material may have a D₁₀ secondary particle diameterwhen measured using solid state powder laser diffraction of at least0.05 μm, or at least 0.1 μm, or at least 0.5 μm, or at least 1 μm. Bymaintaining a D₁₀ particle diameter within these ranges, the potentialfor parasitic reactions in a Li ion cell is reduced from having reducedsurface area, and it is easier to process with less binder in theelectrode slurry. The term “D_(n)” refers to the diameter below which n% by volume of the particle population is found.

The active electrode material may have a Dso secondary particle diameterwhen measured using solid state powder laser diffraction of <50 μm, <20μm, <10 μm, or <5 μm. By maintaining a D₉₀ particle diameter withinthese ranges, the proportion of the particle size distribution withlarge particle sizes is minimised, making the material easier tomanufacture into a homogenous electrode.

The active electrode material may comprise a carbon coating formed onthe surface of the primary crystallites and/or secondary particles. Somesuitable methods for forming carbon coatings on the surface of theprimary crystallites and/or secondary particles are set out inliterature e.g. Zhou (2012).

Other suitable methods are described below. The carbon coating may bepresent in an amount of up to 5 w/w%, based on the total weight of theactive electrode material. The carbon coating may comprise graphiticcarbon.

Where the active electrode has a morphology of a plurality of primarycrystallites where some or all of these primary crystallites areagglomerated into porous secondary particles, the secondary particlesmay comprise a coating of carbon formed at least at the surfaces ofpores in the secondary particles.

In a third aspect, the present invention provides an electrochemicaldevice comprising an anode, a cathode and an electrolyte disposedbetween the anode and the cathode, wherein the anode comprises anelectrode active material according to the first or second aspect of theinvention.

The electrolyte may be a liquid electrolyte. Alternative or additionallythe electrolyte may be a solid state electrolyte.

The anode may further comprise a conductive additive and/or a binder.For example, the anode may have a composition of about 80 wt % activematerial, about 10 wt % conductive additive, and about 10 wt % binder.Alternatively the anode may have a composition of about 91 wt % activematerial, about 5 wt % conductive additive, and about 4 wt % binder. Theamount of active electrode material in the anode may be in a range from70 wt % to 99 wt %, more preferably in a range from 75 wt % to 98 wt %,even more preferably in a range from 85 wt % to 96 wt %.

In a fourth aspect, the present invention provides a use of an electrodeactive material according to the first or second aspect of the inventionas an anode active material, or a component of an anode active material,in an anode in conjunction with a cathode and an electrolyte in: (i) alithium ion battery for charging and discharging of the lithium ionbattery; or (ii) a sodium ion battery for charging and discharging ofthe sodium ion battery.

In a fifth aspect, the present invention provides a method forprocessing an electrode active material according to the first or secondaspects of the invention as or in an anode active material for: (i) alithium ion battery, wherein the method includes diffusing lithium ionsinto the anode active material; or for (ii) a sodium ion battery,wherein the method includes diffusing sodium ions into the anode activematerial.

In a sixth aspect, the present invention provides a method of making anactive electrode material according to the first or second aspect of theinvention, the method comprising steps of:

-   -   providing one or more precursor materials;    -   mixing said precursor materials to form a precursor material        mixture; and    -   heat treating the precursor material mixture in a temperature        range from 400° C.-1350° C. to form the active electrode        material.

Where it is desired to make a material according to the first aspect ofthe invention, preferably the one or more precursor materials includesan M1 ion source, an M2 ion source, and a source of Nb.

Where it is desired to make a material according to the second aspect ofthe invention, preferably, the one or more precursor materials includesa source of M and a source of Nb.

The phrase ‘M1 ion source’ is used herein to describe a materialcomprising M1 ions/atoms. The phrase ‘M2 ion source’ is used herein todescribe a material comprising M2 ions/atoms. For example, the phrase ‘asource of Mo/W/Zr/V/Nb’ is used herein to describe a material comprisingMo/VV/ZrN/Nb ions/atoms, as appropriate.

The precursor materials may include one or more metal oxides, metalhydroxides, metal salts or oxalates. For example, the precursormaterials may include one or more metal oxides of different oxidationstates and/or of different crystal structure. Examples of suitable metaloxide precursor materials include but are not limited to: Nb₂O₅, NbO₂,WO₃, TiO₂, MoO₃, V₂O₅, ZrO₂, and MgO. However, the precursor materialsmay not comprise a metal oxide, or may comprise ion sources other thanoxides. For example, the precursor materials may comprise metal salts(e.g. NO₃ ⁻, SO³⁻) or other compounds (e.g. oxalates). Preferably theone or more precursor materials includes one or more of a Nb source, Mosource, a W source, a Zr source, and/or a V source.

Some or all of the precursor materials may be particulate materials.Where they are particulate materials, preferably they have an averageparticle size of <20 μm in diameter (for example, the D₅₀ diameter whenmeasured using solid state powder laser diffraction). The averageparticle size may be in a range from e.g. 10 nm to 20 μm. Providingparticulate materials with such an average particle size can help topromote more intimate mixing of precursor materials, thereby resultingin more efficient solid-state reaction during the heat treatment step.However, it is not essential that the precursor materials have aninitial particle size of <20 μm in diameter, as the particle size of theone or more precursor materials may be mechanically reduced during thestep of mixing said precursor materials to form a precursor materialmixture.

The step of mixing/milling the precursor materials to form a precursormaterial mixture may be performed by a process selected from (but notlimited to): dry or wet planetary ball milling, rolling ball milling,high shear milling, air jet milling, and/or impact milling. The forceused for mixing/milling may depend on the morphology of the precursormaterials. For example, where some or all of the precursor materialshave larger particle sizes (e.g. an average particle size of greaterthan 20 μm in diameter), the milling force may be selected to reduce theaverage particle size of the precursor materials such that the such thatthe average particle size of the precursor material mixture is reducedto 20 μm in diameter or lower. When the average particle size ofparticles in the precursor material mixture is 20 μm or less, this canpromote a more efficient solid-state reaction of the precursor materialsin the precursor material mixture during the heat treatment step.

The step of heat treating the precursor material mixture may beperformed for a time of from 1 hour to 24 hours, more preferably from 3hours to 14 hours. For example, the heat treatment step may be performedfor 1 hour or more, 2 hours or more, 3 hours or more, 6 hours or more,or 12 hours or more. The heat treatment step may be performed for 24hours or less, 18 hours or less, 14 hours or less, or 12 hours or less.

In some methods it may be beneficial to perform a two-step heattreatment. For example, the precursor material mixture may be heated ata first temperature for a first length of time, follow by heating at asecond temperature for a second length of time. Preferably the secondtemperature is higher than the first temperature. Performing such atwo-step heat treatment may assist the solid state reaction to form thedesired crystal structure.

The step of heat treating the precursor material mixture may beperformed in a gaseous atmosphere. The gaseous atmosphere may be aninert atmosphere, or may be a reducing atmosphere. Where it is desiredto make an oxygen-deficient material, preferably the step of heattreating the precursor material mixture is performed in an inert orreducing atmosphere. Suitable gaseous atmospheres comprise: air, N₂, Ar,He, CO₂, CO, O₂, H₂, and mixtures thereof.

The method may include one or more post-processing steps after formationof the active electrode material.

In some cases, the method may include a post-processing step of heattreating the active electrode material, sometimes referred to as‘annealing’. This post-processing heat treatment step may be performedin a different gaseous atmosphere to the step of heat treating theprecursor material mixture to form the active electrode material. Thepost-processing heat treatment step may be performed in an inert orreducing gaseous atmosphere. Such a post-processing heat treatment stepmay be performed at temperatures of above 500° C., for example at about900° C. Inclusion of a post-processing heat treatment step may bebeneficial to e.g. form deficiencies or defects in the active electrodematerial, for example to form oxygen deficiencies. Advantageously, thepost-processing heat treatment step performed in an inert or reducinggaseous atmosphere may improve the electrical conductivity of the activeelectrode material. In this way, an active electrode material accordingto the second aspect can be synthesised.

In some cases, the method may include a post-processing step of mixingthe active electrode material with a carbon source, and thereby forminga carbon coating on the active electrode material. Optionally, themixture of the active electrode material and the carbon source may beheated to thereby form the carbon coating on the active electrodematerial. Suitable carbon sources include but are not limited to:carbohydrate materials (e.g. sugars, polymers); conductive carbons (e.g.carbon black); and/or aromatic carbon materials (e.g. pitch carbon).

One preferred method of forming a carbon coating includes a step ofmilling the active electrode material with a carbon source, followed bypyrolysis of the active electrode material and carbon source (e.g. in afurnace) under an inert or reducing atmosphere.

Another preferred method of forming a carbon coating includes mixing ofthe active electrode material with a carbon source, dispersion of theactive electrode material and carbon source in an aqueous slurry,followed by spray drying. The resulting powder may optionally bepyrolysed. Where the carbon source is e.g. conductive carbon black, itis not necessary to pyrolyse the material post spray-drying.

In some cases, the method may include a post-processing step of millingthe active electrode material to modify the active electrode materialparticle size. For example, the active electrode material may be treatedby one or more processes including air jet milling, impact milling, highshear milling, sieving, or ball milling. This may provide a moresuitable particle size for use in desired applications of the activeelectrode material.

In a further aspect, the invention provides the use of a dopant M1 forimproving the properties of a base material for use as an activematerial for a metal-ion battery anode, wherein the base material has astructure M2Nb_(y)O_(z) and wherein the dopant is used to provide amodified material [M1]_(x)[M2]_((1-x))[Nb]_(y)[O]_(z), wherein themodified material has improved properties compared to the base material.An improved property may be improved initial coulombic efficiency(exemplified as the difference in the lithiation and de-lithiationcapacity on the 1^(st) charge/discharge cycle at C/10 in a half-cell).An improved property may be improved capacity retention when comparinghigh rate charge/discharging with lower rates (exemplified as 5C or 10Cvs 0.5C). An improved property may be improved specific capacity at lowcharge/discharge rates (exemplified herein at 0.1C). M1, M2, x, y, and zare as defined herein.

The invention includes the combination of the aspects and features andpreferred features described above except where such a combination isclearly impermissible or expressly avoided.

SUMMARY OF THE FIGURES

Embodiments and experiments illustrating the principles of the inventionwill now be discussed with reference to the accompanying figures inwhich:

FIG. 1 shows XRD diffraction patterns of samples 1, 4, 14, 2, 5, 15, 16,18 and 22;

FIG. 2 shows XRD diffraction patterns of samples 8 and 9;

FIG. 3 shows XRD diffraction patterns of samples 6, 7, 17, 19 and 20;

FIG. 4 shows XRD diffraction patterns of samples 10, 11 and 21;

FIG. 5 shows XRD diffraction patterns of samples 12 and 13;

FIG. 6 shows TGA characterisation in air of sample 3;

FIG. 7 shows the particle size distribution of samples 1, 2, 15, and 16;

FIG. 8 shows the particle size distribution of sample 3;

FIG. 9 is an SEM image of sample 3 before pyrolysis and coated withconductive Au for imaging;

FIG. 10 is an SEM image of sample 3 after pyrolysis (no conductivecoating);

FIGS. 11 are SEM images of samples 1 and 2;

FIG. 12 shows representative lithiation and delithiation voltageprofiles obtained by galvanostatic cycling in half cell configuration,1.1-3.0 V voltage window, first 2 cycles at 0.05C rate for samples 1 and16;

FIG. 13 shows representative lithiation and delithiation voltageprofiles obtained by galvanostatic cycling in half cell configuration,1.1-3.0 V voltage window, first 2 cycles at 0.05C rate for samples 6 and7;

FIG. 14 shows lithiation and delithiation capacity obtained bygalvanostatic cycling in half cell configuration, 1.1-3.0 V voltagewindow, at current densities of 0.5C, 10, 2C, 5C (seen as step-changesin the data) for samples 1, 4, and 16;

FIG. 15 shows Lithiation capacity obtained by galvanostatic cycling inhalf cell configuration, 1.1-3.0 V voltage window at current densitiesof 0.5C, 10, 2C, 5C, 0.5C (seen as step changes in the data) for samples6, 7, and 17;

FIGS. 16(a) and (b) show EIS measurements of samples 1, 7, and 16 atdifferent axes scales.

FIG. 17 shows the particle size distributions of sample 16 before andafter post-processing;

FIG. 18 is an SEM image of the surface of an electrode made from sample22, focused on the surface of an active material particle;

FIG. 19 shows representative lithiation and delithiation voltageprofiles obtained by galvanostatic cycling in half cell configuration,1.1-3.0 V voltage window, first 2 cycles at 0.05C rate for samples 12and 13;

FIG. E1 shows XRD diffraction patterns of samples E1, E2.

FIG. E2 shows XRD diffraction patterns of samples E3, E4, E5.

FIG. E3 shows XRD diffraction patterns of samples E6, E7, E8.

FIG. E4 shows XRD diffraction patterns of samples E9, E10.

FIG. E5 shows the particle size distributions of samples E2, E4, E7,E10.

FIG. E6 shows representative lithiation and delithiation voltageprofiles obtained by galvanostatic cycling in half cell configuration,1.1-3.0 V voltage window, first 2 cycles at 0.1C rate for samples E1 andE2.

FIG. E7 shows representative lithiation and delithiation voltageprofiles obtained by galvanostatic cycling in half cell configuration,1.1-3.0 V voltage window, first 2 cycles at 0.1C rate for samples E3 andE5.

FIG. E8 shows representative lithiation and delithiation voltageprofiles obtained by galvanostatic cycling in half cell configuration,1.1-3.0 V voltage window, first 2 cycles at 0.1C rate for samples E6 andE7.

FIG. E9 shows representative lithiation and delithiation voltageprofiles obtained by galvanostatic cycling in half cell configuration,1.1-3.0 V voltage window, first 2 cycles at 0.1C rate for samples E9 andE10. The x axis is in terms of state-of-charge (SOC), to be able tonormalise the curves to their maximum capacities and evaluate the curveshape.

FIG. E10 shows XRD diffraction patterns of E11-E14.

DETAILED DESCRIPTION OF THE INVENTION

Aspects and embodiments of the present invention will now be discussedwith reference to the accompanying figures. Further aspects andembodiments will be apparent to those skilled in the art. All documentsmentioned in this text are incorporated herein by reference.

A number of different materials were prepared and characterised, assummarised in Table 1, below. Broadly, these samples can be split into anumber of groups:

Samples 1, 2, 3, 4, 5, 14, 15, 16, 18, and 22 belong to the same familyof Wadsley-Roth phases based on MoNb₁₂O₃₃(M⁶⁺Nb₁₂O₃₃, 3×4 block ofoctahedra with a tetrahedron at each block corner). The blocks link toeach other by edge sharing between NbO₆ octahedra, as well as cornersharing between M⁶⁺O₄ tetrahedra and NbO₆ octahedra. Sample 1 is thebase crystal structure, which is modified to a mixed metal cationstructure by exchanging one or multiple cations in samples 2 to 4,and/or in a mixed crystal configuration (blending with isostructuralWNb₁₂O₃₃) in samples 14, 15, 16, 18, and 22. Oxygen deficiencies arecreated in the base crystal in sample 5 and in the mixed metal cationstructure 18.

Sample 3 is a spray-dried and carbon-coated version of the crystal madein sample 2, and sample 22 is a spray-dried and carbon-coated version ofthe crystal made in sample 16.

Samples 6, 7, 17, 19, 20 belong to the same family of Wadsley-Rothphases based on ZrNb₂₄O₆₂ (M⁴⁺Nb₂₄O₆₂, 3×4 block of octahedra with halfa tetrahedron at each block corner).

Samples 8, 9 and El 1 belong to the same family of Wadsley-Roth phasesbased on WNb₁₂O₃₃ (M⁶⁺Nb₁₂O₃₃, a 3×4 NbO₆ octahedra block with atetrahedron at each block corner).

Samples 10, 11 and 21 belong to the same family of Wadsley-Roth phasesbased on VNb₉O₂₅ (M⁶⁺Nb₉O₂₅, a 3×3 NbOsoctahedra block with atetrahedron at each block corner).

Samples 12, 13 and E14 belong to the same family of tungsten tetragonalbronzes (TTB) based on W₇Nb₄O₃₁(M⁶⁺ ₇Nb₄O₃₁). This is a tetragonaltungsten bronze structure, where MO₆(M=0.4 Nb+0.6 W) octahedra areexclusively corner-sharing, with 3, 4, and 5 -sided tunnels. Some ofthese tunnels are filled with —O-M-O— chains whereas others are open forlithium ion transport and storage.

Samples E1, E2, E13 belong to the same family of Wadsley-Roth phasesbased on Zn₂Nb₃₄O₈₇ (M²⁺ ₂Nb₃₄O₈₇). This orthorhombic phase consists outof 3×4 blocks of MO₆ octahedra (M═Zn⁺²/Nb+⁵), where the blocks areconnected exclusively by edge-sharing and have no tetrahedra.

Samples E3, E4, E5, E12 belong to the same family of Wadsley-Roth phasesbased on AlNb₁₁O₂₉ (M³⁺Nb₃₄O₈₇). The structure belongs to monoclinicshear structure with 3×4 octahedra blocks connected through exclusivelyedge-sharing and have no tetrahedra.

Samples E6, E7, E8 belong to the same family of Wadsley-Roth phasesbased on GeNb₁₈O₄₇ (M⁴⁺Nb₁₈O₄₇). The structure is similar to sample 10with 3×3 NbO₆ octahedra blocks and one tetrahedron connecting blocks atcorners. However, the structure contains intrinsic defects due to Ge⁺⁴instead of V⁵⁺.

Samples E9, E10 belong to the same family of Wadsley-Roth phases basedon W₅Nb₁₆O₅₅ (M⁶⁺ ₅Nb₁₆O₅₅). The structure is made of 4x5 blocksconnected at the sides by edge-sharing (W,Nb)O₆ and connected at thecorners by Wat tetrahedra. This structure is similar to Sample 8 and 9but with a larger block size.

TABLE 1 A summary of different compositions synthesized. Sample MaterialNo. Composition Synthesis  1 * MoNb₁₂O₃₃ Solid state  2Ti_(0.05)Mo_(0.95)Nb₁₂O₃₃ Solid state  3 Ti_(0.05)Mo_(0.95)Nb₁₂O₃₃ + CSolid state, spray dry, carbon pyrolysis  4 Zr_(0.05)Mo_(0.95)Nb₁₂O₃₃Solid state  5 MoNb₁₂O_(<33) Solid state  6 * ZrNb₂₄O₆₂ Solid state  7V_(0.05)Zr_(0.95)Nb₂₄O₆₂ Solid state  8 * WNb₁₂O₃₃ Solid state  9Ti_(0.05)W_(0.95)Nb₁₂O₃₃ Solid state 10 * VNb₉O₂₅ Solid state 11Ti_(0.05)V_(0.95)Nb₉O₂₅ Solid state 12 * W₇Nb₄O₃₁ (WNb_(0.57)O_(4.43))Solid state 13 Ti_(0.05)W_(0.95)Nb_(0.57)O_(4.43)(Ti_(0.35)W_(6.65)Nb₄O₃₁) Solid state 14 W_(0.25)Mo_(0.75)Nb₁₂O₃₃ Solidstate 15 Ti_(0.05)W_(0.25)Mo_(0.70)Nb₁₂O₃₃ Solid state 16Ti_(0.05)Zr_(0.05)W_(0.25)Mo_(0.65)Nb₁₂O₃₃ Solid state 17Ti_(0.05)Zr_(0.95)Nb₂₄O₆₂ Solid state 18Ti_(0.05)Zr_(0.05)W_(0.25)Mo_(0.65)Nb₁₂O_(<33) Solid state 19Mo_(0.05)Zr_(0.95)Nb₂₄O₆₂ Solid state 20Mo_(0.05)V_(0.05)Zr_(0.95)Nb₂₄O₆₂ Solid state 21 Mo_(0.05)V_(0.95)Nb₉O₂₅Solid state 22 Ti_(0.05)Zr_(0.05)W_(0.25)Mo_(0.65)Nb₁₂O₃₃ + C Solidstate, spray dry, carbon pyrolysis E1* Zn₂Nb₃₄O₈₇ Solid state E2Ge_(0.1)Zn_(1.9)Nb₃₄O₈₇ Solid state E3* AlNb₁₁O₂₉ Solid state E4Fe_(0.05)Al_(0.95)Nb₁₁O₂₉ Solid state E5 Ga_(0.05)Al_(0.95)Nb₁₁O₂₉ Solidstate E6* GeNb₁₈O₄₇ Solid state E7 K_(0.02)Co_(0.02)Ge_(0.96)Nb₁₈O₄₇Solid state E8 K_(0.02)Co_(0.02)Ge_(0.96)Nb₁₈O_(47-α) Solid state E9*W₅Nb₁₆O₅₅ Solid state E10 W₅Nb₁₆O_(55-α) Solid state E11 WNb₁₂O_(33-α)Solid state E12 AlNb₁₁O_(29-α) Solid state E13 Zn₂Nb₃₄O_(87-α) Solidstate E14 W₇Nb₄O_(31-α) Solid state Samples indicated with * arecomparative samples.

Material Synthesis

Samples listed in Table 1 were synthesised using a solid-state route. Ina first step, metal oxide precursor commercial powders (Nb₂O₅, NbO₂,MoO₃, ZrO₂, TiO₂, W03, V205, ZrO₂, K2O, CoO, Fe2O₃, GeO₂, Ga2O₃, Al₂O₃,ZnO and/or MgO) were mixed in stochiometric proportions and planetaryball-milled at 550 rpm for 3h in a zirconia jar and milling media with aball to powder ratio of 10:1. The resulting powders were then heated ina static muffle furnace in air in order to form the desired crystalphase. Samples 1 to 5 and 12 to 16, 18 and 22 were heat-treated at 900°C. for 12h; samples 6 to 9, 17, 19, and 20 were heat-treated at 1200° C.for 12h, with samples 6, 7, 17, 19 and 20 undergoing a further heattreatment step at 1350° C. for an additional 4h; samples 10, 11 and 21were heat-treated at 1000° C. for 12h. Sample 3 and 22 were furthermixed with a carbohydrate precursor (such as sucrose, maltodextrin orother water-soluble carbohydrates), dispersed in an aqueous slurry atconcentrations of 5, 10, 15, or 20 w/w% with ionic surfactant, andspray-dried in a lab-scale spray-drier (inlet temperature 220° C.,outlet temperature 95° C., 500 mL/h sample introduction rate). Theresulting powder was pyrolyzed at 600° C. for 5h in nitrogen. Sample 5and 18 were further annealed in nitrogen at 900° C. for 4 hours.

Samples E1, E2, E6, E7, E8, E9, E10 were prepared by ball milling asabove, and impact milling at 20,000 rpm as needed to a particle sizedistribution with D90<20 μm, then heat-treated as in a muffle furnace inair at 1200° C. for 12 h; samples E8, E10, E11, E12, E13 were furtherannealed in nitrogen at 1000° C. for 4 h; E14 was annealed in nitrogenat 900° C. for 5 h. Samples E3, E4, E5 were heat-treated at 1300° C. for12 h. Samples E1-E10 were de-agglomerated after synthesis by impactmilling or jet milling to the desired particle size ranges.

Elemental Analysis of Samples

Elemental analysis was carried out by Inductively-Coupled Plasma-OpticalEmission Spectroscopy (ICP-MS/OES). The measurements were carried out ona Thermo Scientific ICP-OES Duo iCAP 7000 series. The samples weredigested using 5 ml Nitric acid and 1 ml HF acid and an internalstandard was used to account for any instrumental variation. In thisprocess the plasma is used to vaporise the material into itsatomic/ionic state of elements. The atoms are in excited state due tohigh temperature and the decay to normal state through energytransitions. The characteristic radiation emitted by each excited ion ismeasured for analysis. The results are set out in Table 2, below.

TABLE 2 Summary of ICP-OES elemental analysis results for samples 1, 2,4, 14, 3, 16, 9, 11, and 17 Sample Composition Elemental ratio ExpectedMeasured  1* MoNb₁₂O₃₃ Nb/Mo 12 12  2 Ti_(0.05)Mo_(0.95)Nb₁₂O₃₃ Mo/Ti 1918  4 Zr_(0.05)Mo_(0.95)Nb₁₂O₃₃ Mo/Zr 19 18 14 W_(0.25)Mo_(0.75)Nb₁₂O₃₃Mo/W  3   3.1  3 Ti_(0.05)Mo_(0.95)Nb₁₂O₃₃ + C Mo/Ti 19 18 16Ti_(0.05)Zr_(0.05)W_(0.25)Mo_(0.65)Nb₁₂O₃₃ Mo/Zr; Mo/Ti 13; 13 11.4;13.5  9 Ti_(0.05)W_(0.95)Nb₁₂O₃₃ W/Ti 19 18 11 Ti_(0.05)V_(0.95)Nb₉O₂₅V/Ti 19 19 17 Ti_(0.05)Zr_(0.95)Nb₂₄O₆₂ Zr/Ti 19 19

This table of elemental analysis demonstrates that substantially theexpected cation ratio has been achieved for each composition tested.

XRD Characterisation of Samples

The phase purity of some samples was analysed using Rigaku Miniflexpowder X-ray diffractometer in 2 θ range (10-70°) at 1°/min scan rate.

FIG. 1 shows the measured XRD diffraction patterns for samples 1, 4, 14,2, 5, 15, 16, 18, 22 which are relevant to Comparative Study A. Alldiffraction patterns have peaks at the same locations (within instrumenterror, that is 0.1°), and match JCPDS crystallography database entryJCPDS 73-1322, which corresponds to MoNb₁₂O₃₃. There is no amorphousbackground noise and the peaks are sharp and intense. This means thatall samples are phase-pure and crystalline, with crystallite size ˜200nm according to the Scherrer equation and crystal structure matchingMoNb₁₂O₃₃.

FIG. 2 shows the measured XRD diffraction patterns for samples 8 and 9.FIG. E10 shows the XRD pattern for sample E11. All diffraction patternshave peaks at the same locations (within instrument error, that is0.1°), and match JCPDS crystallography database entry JCPDS 73-1322,which corresponds to WNb₁₂O₃₃. There is no amorphous background noiseand the peaks are sharp and intense. This means that all samples arephase-pure and crystalline, with crystallite size ˜200 nm according tothe Scherrer equation and crystal structure matching WNb₁₂O₃₃.

FIG. 3 shows the measured XRD diffraction patterns for samples 6, 7, 17,19, 20 which are relevant to Comparative Study B. All diffractionpatterns have peaks at the same locations (within instrument error, thatis 0.1°), and match JCPDS crystallography database entry JCPDS01-072-1655, which corresponds to ZrNb₂₄O₆₂. There is no amorphousbackground noise and the peaks are sharp and intense. This means thatall samples are phase-pure and crystalline, with crystallite size ˜200nm according to the Scherrer equation and crystal structure matchingZrNb₂₄O₆₂.

FIG. 4 shows the measured XRD diffraction patterns for samples 10, 11,21. All diffraction patterns have peaks at the same locations (withininstrument error, that is 0.1°), and match JCPDS crystallographydatabase entry JCPDS 00-049-0289, which corresponds to VNb₉O₂₅. There isno amorphous background noise and the peaks are sharp and intense. Thismeans that all samples are phase-pure and crystalline, with crystallitesize ˜200 nm according to the Scherrer equation and crystal structurematching VNb₉O₂₅.

FIG. 5 shows the measured XRD diffraction patterns for samples 12 and13. FIG. E10 shows the XRD pattern for sample E14. All diffractionpatterns have peaks at the same locations (within instrument error, thatis 0.1°), and match JCPDS crystallography database entry JCPDS00-020-1320, which corresponds to W₇Nb₄O₃₁. There is no amorphousbackground noise and the peaks are sharp and intense. This means thatall samples are phase-pure and crystalline, with crystallite size ˜200nm according to the Scherrer equation and crystal structure matchingW₇Nb₄O₃₁.

FIG. E1 shows the measured XRD diffraction patterns for samples E1, E2.FIG. E10 shows the XRD pattern for sample E13. All diffraction patternshave peaks at the same locations (within)0.1-0.2°, and match JCPDScrystallography database entry JCPDS 22-353. There is no amorphousbackground noise and the peaks are sharp and intense. This means thatall samples are phase-pure and crystalline, with crystallite size 52±12nm according to the Scherrer equation and crystal structure matchingZn₂Nb₃₄O₈₇.

FIG. E2 shows the measured XRD diffraction patterns for samples E3, E4,E5. FIG. E10 shows the XRD pattern for sample E12. All diffractionpatterns have peaks at the same locations (within)0.1-0.2°, and matchJCPDS crystallography database entry JCPDS 72-159 (isostructuralTi₂Nb₁₀O₂₉). There is no amorphous background noise and the peaks aresharp and intense. This means that all samples are phase-pure andcrystalline, with crystallite size 53±16 nm according to the Scherrerequation and crystal structure matching AlNb₁₁O₂₉.

FIG. E3 shows the measured XRD diffraction patterns for samples E6, E7,E8. All diffraction patterns have peaks at the same locations(within)0.1-0.2° , and match ICSD crystallography database entry 72683(isostructural PNb₉O₂₅). There is no amorphous background noise and thepeaks are sharp and intense. This means that all samples are phase-pureand crystalline, with crystallite size 53±3 nm according to the Scherrerequation and crystal structure matching GeNb₁₈O₄₇.

FIG. E4 shows the measured XRD diffraction patterns for samples E9, E10.All diffraction patterns have peaks at the same locations(within)0.1-0.2°, and match JCPDS crystallography database entry JCPDS44-0467. There is no amorphous background noise and the peaks are sharpand intense. This means that all samples are phase-pure and crystalline,with crystallite size 37±11 nm according to the Scherrer equation andcrystal structure matching W5Nbi6O_(55.)

TGA Characterisation of Samples

Thermogravimetric Analysis (TGA) was performed on some samples using aPerkin Elmer Pyris 1 system in a synthetic air atmosphere. Samples werefirst held for 15 min at 30° C., then heated from 30° C. to 950° C. at5° C./min, and finally held for 30 min at 950° C. TGA was performed onsample 3 to quantify carbon content, and on sample 5 to show massincrease as oxygen vacancies are filled.

FIG. 6 shows TGA characterisation in air of sample 3. The sharp drop inmass between ˜400° C. and 500° C. is attributed to the decomposition ofthe carbon coating. The decomposition temperature corresponds to amixture of amorphous and graphitic carbon. The amount of mass lossindicates that sample 3 includes 1.1 w. % of carbon coating, which is inline with the amount expected from the stoichiometry of the precursors.

Qualitative Assessment of Oxygen Deficiency

As discussed above, sample 5 and 18 were heat-treated at 900° C. for 12h to form the active electrode material, and was then further annealedin nitrogen (a reducing atmosphere) at 900° C., in a post-processingheat treatment step. A colour change from white to dark purple wasobserved after the post-processing heat treatment in nitrogen,indicating change in oxidation states and band structure of thematerial, as a result of oxygen deficiency of the sample.

Samples E8, E10, E11, E12, E13 were further annealed in nitrogen at1000° C. for 4 h, sample E14 was annealed in nitrogen at 900° C. for 5h. Sample E7 transitions from a white colour to a deep yellow colourupon introduction of induced oxygen deficiencies in sample E8; sample E9transitions from an off-white colour to a blue-grey colour uponintroduction of induced oxygen deficiencies in sample E10; sample 8transitions from off-white to light blue in E11; sample E3 transitionsfrom white to grey/black in E12; sample E1 transitions from white togrey/black in E3; sample 12 transitions from light yellow to dark bluein E14.

Particle Size Distribution Analysis of Samples

Particle Size Distributions were obtained with a Horiba laserdiffraction particle analyser for dry powder.

Air pressure was kept at 0.3 MPa. The results are set out in Table 3,below.

TABLE 3 Summary of particle size distribution statistics for samples 1,2, 15, 16, 18, 3 before pyrolysis, 3 after pyrolysis, 16 and 18 afterpost-processing, and samples E1-E14. D₁₀ D₅₀ D₉₀ Sample [μm] [μm] [μm] 1* 3.8 11.2 50.0  2 2.6 10.9 87.4 15 3.6 21.2 55.3 16 4.7 31.2 82.9 185.1 57.7 176 3 before pyrolysis 4.2 8.2 16.3 3 after pyrolysis 6.7 12.751.1 16 after impaction 1.0 2.6 4.8 milling 18 after impaction 1.4 4.49.6 milling E1* 3.7 5.9 9.3 E2 5.1 9.2 16.5 E3* 3.6 6.6 12.0 E4 4.3 7.713.9 E5 3.7 7.0 15.5 E6* 4.3 8.1 16.5 E7 4.3 9.7 20.4 E8 5.3 10.8 21.3E9* 3.1 5.5 9.3 E10 2.7 5.1 9.3 E11 3.3 5.5 8.7 E12 4.2 7.8 18.4 E13 4.26.8 10.8 E14 1.2 4.5 10.1

FIG. 7 shows particle size distributions (measured particle size beingsecondary particle size, not crystal or crystallite size) for samples 1,2, 15, and 16, as a representative example of particle sizedistributions obtained by solid state routes in this study withoutfurther processing or size optimisation. The particle size distributionsare typically bi-modal, with a first mode ˜10 μm, and a second mode ˜90μm. Sample 3 presents significant differences in terms of particle sizedistribution, as shown in FIG. 8 due to the spray-drying and pyrolysispost-processing step.

All particle size distributions can also be refined with furtherprocessing steps, for example spray drying, ball milling, high shearmilling, jet milling or impact milling to reduce the particle sizedistribution to the desired range (e.g. d90 <20 μm, <10 μm or <5 μm) asshown in FIG. 17 and Table 3. Typically the particle size distributionsare tuned by optimising the phase formation process (i.e. solid statesynthesis route) and post-processing steps for the target application.For example, for a Li ion electrode with high power, one would typicallytarget lower average particle sizes, amongst other considerations.

FIG. E5 shows the particle size distributions for samples E2, E4, E7,E10 in their final form, which are then processed into electrodeslurries and inks.

SEM Characterisation of Samples

The morphology of some samples was analysed by Scanning ElectronMicroscopy (SEM).

FIGS. 9 and 10 show SEM images of sample 3 before and after pyrolysis. Aporous microsphere morphology with carbon coating is observed, withprimary crystallites organised into secondary particles. It can be seenthat the material has with homogeneous porous particles that can packefficiently to form a high-density electrode. Qualitatively theconductivity is vastly improved as a conductive coating does not need tobe applied for SEM imaging to be carried out, implying an order ofmagnitude improvement in material surface conductivity. FIG. 18 is anSEM image of the surface of a particle in an electrode of sample 22,where conductive carbon black particles contained in the electrode canalso be seen in the right side of the image. This visibly shows evidenceof a conformal carbon coating around the MNO material.

FIG. 11 shows SEM images of samples 1 and 2, and corroborates XRD andPSD data, showing compact secondary particle micron-size particlescomposed of ˜200 nm primary crystallites.

Electrochemical Testing of Samples

Electrochemical tests were carried out in half-coin cells (CR2032 size)for initial analysis. In half-coin tests, the material is tested in anelectrode versus a Li metal electrode to assess its fundamentalperformance. In the below examples, the active material composition tobe tested was combined with N-Methyl Pyrrolidone (NMP), carbon blackacting as a conductive additive, and poly(vinyldifluoride) (PVDF) binderand mixed to form a slurry using a lab-scale centrifugal planetary mixer(although it is also possible to form aqueous slurries by using waterrather than NMP). The non-NMP composition of the slurries was 80 w. %active material, 10 w. % conductive additive, 10 w. % binder. The slurrywas then coated on an Al foil current collector to the desired loadingof 1 mg/cm² by doctor blade coating and dried in a vacuum oven for 12hours. Electrodes were punched out at the desired size and combined witha separator (Celgard porous PP/PE), Li metal, and electrolyte (1 M LiPF₆in EC/DEC) inside a steel coin cell casing and sealed under pressure.Formation cycling was then carried out at low current rates (C/20) for 2full charge and discharge cycles. After formation, further cycling canbe carried out at a fixed or varied current density as required. Thesetests have been termed “half-cell galvanostatic cycling” for futurereference. For samples E1-E10, the electrolyte was altered to 1.3 MLiPF6 in 3:7 EC/DEC, and the formation cycling was carried out at C/10for 2 charge/discharge cycles in the limits 1.1-3.0 V. The values shownfor these samples is an average of 3 measurements, with the error beingthe standard deviation.

Homogeneous, smooth coatings on current collector foil, the coatingsbeing free of visible defects were also prepared as above with acentrifugal planetary mixer to a composition of 94 w. % active material,4 w. % conductive additive, 2 w. % binder. The coatings were calendaredat 80° C. to a density of up to 3.0 g/cm³ at loadings of 1.3-1.7 mAh/cm²in order to demonstrate possible volumetric capacities >700 mAh/cm³ inthe voltage range 0.7-3.0 V at C/20, and >640 mAh/cm³ in the voltagerange 1.1-3.0 V at C/5. This is an important demonstration of thesematerials being viable in a commercially focussed electrode power cellformulation, where retaining performance after calendaring to a highelectrode density allows for high volumetric capacities. Loadings of upto and including 1.0, 1.5, 2.0, 2.5, or 3.0 mAh/cm² may be useful forLi-ion cells focussed on power performance; loadings greater than 3.0,4.0, or 5.0 mAh/cm² are useful for energy-focussed performance in Li ioncells. Calendaring of these materials was demonstrated down to electrodeporosity values of 35%, and typically in the range 35-40%; defined asmeasured electrode density divided by the average of the true densitiesof each electrode component adjusted to their w/w%.

Electrical conductivity of electrodes made with the samples listed inTable 1 was measured using a 4-point probe thin film resistancemeasurement apparatus. Slurries were formulated according to theprocedure described above and coated on a dielectric mylar film at aloading of 1 mg/cm². Electrode-sized discs where then punched out andresistance of the coated-film was measured using a 4-point probe. Bulkresistivity can be calculated from measured resistance using thefollowing equation:

Bulk resistivity (ρ)=2πs(V/I); R=V/I; s=0.1 cm =2λx0.1xR (Ω)   (3)

The results of this test are shown in Table 4, below:

TABLE 4 Summary of 4-point probe resistivity measurement results forsamples 1, 2, 4, 5, 6, 7, 13 to 20, and 22. Resistance Bulk resistivitySample [kΩ] [kΩ · cm]  1* 8.5 5.3  2 1.7 1.1  4 3.2 2.0  5 0.52 0.33  6*0.37 0.23  7 0.52 0.33 13 0.45 0.28 14 2.7 1.7 15 1.2 0.75 16 1.3 0.8217 0.34 0.21 18 0.89 0.56 19 0.18 0.11 20 0.20 0.13 22 0.33 0.21

Samples E1-E14 also had their 4-point probe resistance measured toquantify their electrical resistivity. This was carried out with adifferent Ossila instrument (T2001A3-UK) at 23° C. for coatings on mylarfilms at loadings of 1.0 mg/cm². The results for sheet resistance(Ω/square) are outlined in Table 4a, with error based on the standarddeviation of 3 measurements.

TABLE 4a Summary of 4-point probe resistivity measurement results forsamples E1 to E14. Sample Sheet Resistivity [Ω/square] E1*  1242 ±156 E2 1041 ± 103 E3* 1396 ± 74 E4 1215 ± 52 E5 1057 ± 35 E6* 1092 ± 52 E71009 ± 89 E8  965 ± 83 E9* 1135 ± 92 E10 1113 ± 99 E12  891 ± 61 E131027 ± 13 12*  853 ± 51 E14  846 ± 57  6*  880 ± 29

The direct current internal resistance (DCIR) and the resultant areaspecific impedance (ASI) is a key measurement of internal resistance inthe electrode in a Li-ion cell. In a typical measurement, a cell thathas already undergone formation will be cycled at C/2 for 3 cycles. Withthe electrode in its delithiated state a C/2 discharge current isapplied for 1 h to achieve ˜50% lithiation. The cell is rested for 30mins to equilibrate at its OCV (open circuit voltage), and then a 5Ccurrent pulse is applied for 10 s, followed by a 30 mins rest to reachthe OCV. During the 10 s pulse the voltage response is sampled at ahigher frequency to determine the average internal resistanceaccurately. The resistance is then calculated from V=IR, using thedifference between the OCV (the linear average between the initial OCVbefore the pulse and afterwards) and the measured voltage. Theresistance is then multiplied by the area of the electrode to result inthe ASI.

The results of this test are shown in Table 5, below:

TABLE 5 Summary of DCIR/ASI measurement results for samples 1, 2, 4, 7,14, 16, and 17. Sample ASI/Ω · cm²  1* 141  2 125  4 120  6* 126  7 16213 67 14 99 16 74 17 162 18 75 19 164 22 121

The reversible specific capacity 0/20, initial coulombic efficiency,nominal lithiation voltage vs Li/Li+ at C/20, 5C/0.5C capacityretention, and 100/0.50 capacity retention for a number of samples werealso tested, the results being set out in Table 6, below. Nominallithiation voltage vs Li/Li+ has been calculated from the integral ofthe V/Q curve divided by the total capacity on the 2^(nd) cycle 0/20lithiation. Capacity retention at 10C and 5C has been calculated bytaking the specific capacity at 10C or 5C, and dividing it by thespecific capacity at 0.5C. It should be noted that the capacityretention was tested with symmetric cycling tests, with equivalentC-rate on lithiation and de-lithiation. Upon testing with an asymmetriccycling program, 100/0.50 capacity retention greater than 89% isroutinely observed.

Samples E1-E10 were tested with minor differences in Table 6a, thereversible specific capacity shown is the 2^(nd) cycle delithiationcapacity at 0/10, the nominal lithiation voltage vs Li/Li+ is at C/10 inthe 2^(nd) cycle, the rate tests were carried out with an asymmetriccycling program with no constant voltage steps (i.e. constant current),with lithiation at C/5 and delithiation at increasing C-rates.

TABLE 6 Summary of electrochemical testing results from Li-ion half coincells using a number of samples. In general (although not exclusively)it is beneficial to have a higher capacity, a higher ICE, a lowernominal voltage, and higher capacity retentions. Reversible Nominalspecific Initial lithiation 5 C/0.5 C 10 C/0.5 C capacity coulombicvoltage vs capacity capacity C/20 efficiency Li/Li⁺ retention retentionSample [mAh/g] [%] [V] [%] [%]  1* 214 87.8 1.61 62 35  2 240 90.9 1.6164 45  3 203 84.9 1.58 79 68  4 286 90.7 1.59 68 54  5 253 86.0 1.60 6343  6* 224 93.5 1.57 61 38  7 263 93.6 1.58 74 67  8* 192 82.0 1.60 5436  9 188 86.8 1.61 64 54 10* 172 74.3 1.55 64 54 11 176 71.6 1.59 56 4512* 164 93.9 1.77 86 81 13 184 95.4 1.75 86 80 14 278 91.0 1.59 15 22889.2 1.59 16 281 90.8 1.58 72 58 17 203 94.6 1.58 18 228 90.1 1.59 84 6819 193 87.0 1.56 63 44 21 169 70.9 1.59 67 56 22 267 86.9 1.57 71 62

TABLE 6a Summary of electrochemical testing results from Li-ion halfcoin cells using a number of samples. Nominal Specific Initiallithiation 5 C/0.5 C 10 C/0.5 C capacity C/10 coulombic voltage C/10 ASIcapacity capacity Sample [mAh/g] efficiency [%] [V] [Ω · cm²] retention[%] retention [%] E1* 222 ± 7  98.23 ± 0.51 1.543 ± 0.001 169 ± 10 96.5± 0.1 95.9 ± 0.1 E2 273 ± 17 98.52 ± 0.45 1.550 ± 0.001 106 ± 18 97.3 ±0.4 96.2 ± 0.7 E3* 244 ± 26 96.75 ± 0.31 1.549 ± 0.002 166 ± 17 96.1 ±0.6 95.2 ± 0.8 E4 252 ± 9  98.80 ± 0.86 1.549 ± 0.001 109 ± 9  98.4 ±0.0 97.4 ± 0.1 E5 272 ± 21 99.69 ± 1.56 1.549 ± 0.001 122 ± 3  96.3 ±0.3 94.8 ± 0.4 E6* 134 ± 14 80.97 ± 1.55 1.539 ± 0.007 485 ± 75 72.8 ±5.7 64.1 ± 7.2 E7 150 ± 8  82.15 ± 0.12 1.531 ± 0.000 390 ± 32 67.0 ±0.4 56.8 ± 0.5 E8 144 ± 2  81.64 ± 1.35 1.530 ± 0.001 400 ± 42 72.9 ±1.2 63.3 ± 1.5 E9* 211 ± 5  94.53 ± 0.18 1.630 ± 0.001 129 ± 13 96.2 ±0.4 95.1 ± 0.5 E10 201 ± 7  98.42 ± 1.12 1.626 ± 0.000 118 ± 16 96.2 ±0.1 94.9 ± 0.2 E12 198 ± 13 97.71 ± 0.25 1.544 ± 0.001 208 ± 8  95.2 ±0.8 92.9 ± 1.0 E13 203 ± 15 98.22 ± 0.12 1.546 ± 0.001 199 ± 10 97.7 ±0.0 97.7 ± 0.5

The modification of mixed niobium oxide-based Wadsley-Roth and Bronzestructures as outlined in the claims demonstrate the applicability ofthe present invention to improve active material performance in Li-ioncells. By substituting the non-Nb cation to form a mixed cationstructure as described, the entropy (cf disorder) can increase in thecrystal structure, reducing potential energy barriers to Li iondiffusion through minor defect introduction (e.g. samples E7, 16).Modification by creating mixed cation structures that retain the sameoverall oxidation state demonstrate the potential improvements byaltering ionic radii, for example replacement of an Mo⁶⁺ cation with W⁶⁺in sample 14 or Fe³⁺ or Ga³⁺ for Al³⁺ in samples E4 and E5, which cancause minor changes in crystal parameters and Li-ion cavities (e.g.tuning the reversibility of Type VI cavities in Wadsley-Roth structures)that can improve specific capacity, Li-ion diffusion, and increaseCoulombic efficiencies of cycling by reducing Li ion trapping.Modification by creating mixed cation structures that result inincreased oxidation state (e.g. Ge⁴⁺ to replace Zn²⁺ in sample E2, orMo⁶⁺ for Zr⁴⁺ in sample 19) demonstrate similar potential advantageswith altered ionic radii relating to capacity and efficiency, compoundedby introduction of additional electron holes in the structure to aid inelectrical conductivity. Modification by creating mixed cationstructures that result in decreased oxidation state (e.g. K+ and Co³⁺ toreplace Ge⁴⁺ in sample E7, or Ti⁴⁺ to replace Mo⁶⁺ in sample 2)demonstrate similar potential advantages with altered ionic radiirelating to capacity and efficiency, compounded by introduction ofoxygen vacancies and additional electrons in the structure to aid inelectrical conductivity. Modification by inducing oxygen deficiency fromhigh temperature treatment in inert or reducing conditions demonstratethe loss of a small proportion of oxygen from the structure, providing areduced structure of much improved electrical conductivity (e.g. sample5, E10 and E12-14) and improved electrochemical properties such ascapacity retention at high C-rates (e.g. sample 5, E13). Combination ofmixed cation structures and induced oxygen deficiency allows multiplebeneficial effects (e.g. increased specific capacity, reduced electricalresistance) to be compounded (e.g. samples 18, E8).

FIGS. 12, 13, and 19 show representative lithiation/delithiation curvesfor unmodified and modified MoNb₁₂O₃₃ (FIG. 12 —samples 1 and 6)ZrNb₂₄O₆₂ (FIG. 13 —samples 6 and 7), and W₇Nb₄O₃₁ (FIG. 19 —samples 12and 13) in their first two formation cycles at C/20 rate. In FIG. 12 ,approximately 90% of the specific capacity for sample 16 demonstrated isshown to be in a narrow voltage range of ca. 1.2—2.0 V, and in FIG. 13approximately 90% of the capacity for sample 7 demonstrated is shown tobe in a narrow range of ca. 1.25-1.75 V; these data highlight theattractive voltage profiles achievable with MNO crystals based uponWadsley-Roth crystal structures. In FIG. 19 , approximately 90% of thespecific capacity for sample 13 is shown to be in a narrow range of ca.1.2-2.2 V; this demonstrates that attractive voltage profiles areachieved with MNO crystals based upon a tetragonal bronze crystalstructure. Secondly, the complex metal oxide samples 7, 16, and 13demonstrate improved specific capacity as compared to their unmodifiedcrystals samples 1, 6 and 12. This is due to the cations that areincluded in the complex structures increasing the number of sites in thecrystal that Li ions can accommodate due to their differing ionic radiiand oxidation states, thus increasing capacity. An increase in ICE wasobserved between samples 1 and 16, and samples 12 and 13, which furtherdemonstrates that Li ions intercalated in the modified crystal structurecan be more efficiently delithiated as the Li ion sites are modified toenable their de-intercalation.

FIG. E5 demonstrates the particle size distribution of samples E2, E4,E8, E11 containing primarily a single peak that has a narrowdistribution, i.e. D₁₀ and D₉₀ are similar in value to D₅₀. This isadvantageous for processing the material in electrode slurries forefficient packing of the material, and to maintain a homogeneouselectrochemical performance (e.g. a smaller particle will be fullylithiated in advance of a larger particle due to shorter diffusiondistances).

FIG. E6 shows the advantage in modifying sample E1, particularly withregard to improving the observed specific capacity through substitutingZn²⁺ cations with Ge⁴⁺ cations of higher valency. FIG. E7 demonstratesthe improved specific capacity observed on modifying sample E6 bysubstituting Ge4+ with K and Co cations, i.e. with cations of reducedvalency. FIG. E9 demonstrates the improvement in ICE, and reduction innominal lithiation voltage possible by introduction of induced oxygenvacancies that reduces polarisation effects through improvingconductivity, and through improving the reversibility oflithiation/delithiation processes.

Across all materials tested, each material according to the inventiondemonstrates an improvement versus the unmodified ‘base’ crystalstructure. This is inferred from measurements of resistivity/impedanceby two different methods, and also electrochemical tests carried out inLi-ion half coin cells, particularly the capacity retention at increasedcurrent densities (cf. rates, Table 6, FIGS. 14 and 15 ). Withoutwishing to be bound by theory, the inventors suggest that this is aresult of increased ionic and electronic conductivity of the materialsas defects are introduced, or by alterations to the crystal lattice byvarying ionic radii; also evidenced by DCIR/ASI (Table 5) and EIS (FIG.16 ) measurements to show decreased resistance or impedance uponmaterial modification. Li-ion diffusion rates likely also increase inmaterials according to the invention, as compared with the unmodified‘base’ materials. Specific capacities themselves may also increase insome cases as shown in Table 6, as doping/exchange with metal ions ofdifferent sizes can expand or contract the crystal lattice and allow formore intercalation or more reversibility of intercalation of Li-ionsthan possible in the unmodified structure.

The data in Table 4 show a large reduction in the resistivity betweensample 1 (comparative) and samples 2, 4, 5, 14, 15, 16, 18, 22,demonstrating the effect of embodiments of the present invention onimproving electrical conductivity of the crystal structures through bothcation exchange, oxygen deficiencies, and carbon coating. Samples 17,19, and 20 also show a similarly low resistivity versus sample 6. Theresistivity slightly increased upon incorporation of 0.05 equivalents ofV species in the base crystal in sample 7, however an improvement inspecific capacity was observed due to the changes in available Li-ionsites in the crystal lattice likely as a result of the differing ionicradius of V over Zr (see Table 6).

The data in Table 5 shows a large reduction in the DCIR/ASI from sample1 (comparative) to samples 2, 4, 14, 16, 18 and 22, reflecting thetrends shown in Table 4. Samples 7, 17, and 19 demonstrate a higher thanthese by DCIR, however these relate to a different base crystalstructure. Without wishing to be bound by theory, the inventorshypothesise that samples 7, 17, and 19 demonstrate an increase inDCIR/ASI as compared with the comparative material of sample 6(ZrNb₂₄O₆₂) due to the changes in the crystal lattice with theintroduced cations of different ionic radii. However, it remainsbeneficial in terms of conductivity for these structures for samples 17and 19 as the electrical resistivity is decreased as shown in Table 4,thereby minimising joule heating and enabling a more uniform currentdistribution across the material, which in turn can enable improvedsafety and lifetime of a Li ion system. For sample 7, whilst there is nodemonstrated improvement utilising V to exchange with Zr, there is anincrease in specific capacity, as discussed above.

In Table 6, across most samples there is a trend for improved specificcapacities, initial Coulombic efficiencies (ICE), nominal lithiationvoltage vs Li/Li+, and importantly capacity retention at 5C and 10C vs0.5C for materials according to the invention versus the comparative‘base’ materials (e.g. samples 1, 6, 8, 10, 12). For example samples 2,3, 4, 5, 14, 15, 16, 18, 22 all demonstrate improvements in one or moreof these parameters vs sample 1. This is also the case for samples 7,17, 19 versus sample 6 across multiple parameters; sample 11 and 21versus 10 where an improvement in specific capacity or capacityretention is observed; sample 9 versus 8 where ICE and capacityretention are improved; and sample 13 versus 12* where there areimprovements in all parameters.

FIGS. 14 and 15 demonstrate improved capacity retention at highercycling rates for materials according to the invention (samples 4, 16,7, 17) versus the comparative materials (samples 1 and 6).

Electrochemical impedance spectroscopy (EIS) measurements were alsocarried out to gain a further understanding on the impedance present inthe electrode in a Li-ion cell. In a typical measurement, the cell isprepared as for DCIR measurements to ˜50% lithiation and then thefrequency of alternating charge/discharge current pulses is variedwhilst measuring the impedance. By plotting the real and imaginarycomponents as the axes, and varying the AC frequency, a Nyquist plot isgenerated. From this plot for a Li-ion cell different types of impedancein the cell can be identified, however it is typically complex tointerpret. For example, Ohmic resistance can be partially separated fromelectrochemical double layer effects and also separated from diffusioneffects.

FIG. 16(a) and (b) show EIS spectra for (comparative) sample 1 andsamples 16 and 7 (samples according to the invention).

The features disclosed in the foregoing description, or in the followingclaims, or in the accompanying drawings, expressed in their specificforms or in terms of a means for performing the disclosed function, or amethod or process for obtaining the disclosed results, as appropriate,may, separately, or in any combination of such features, be utilised forrealising the invention in diverse forms thereof.

While the invention has been described in conjunction with the exemplaryembodiments described above, many equivalent modifications andvariations will be apparent to those skilled in the art when given thisdisclosure. Accordingly, the exemplary embodiments of the invention setforth above are considered to be illustrative and not limiting. Variouschanges to the described embodiments may be made without departing fromthe spirit and scope of the invention.

For the avoidance of any doubt, any theoretical explanations providedherein are provided for the purposes of improving the understanding of areader. The inventors do not wish to be bound by any of thesetheoretical explanations.

Any section headings used herein are for organizational purposes onlyand are not to be construed as limiting the subject matter described.

Throughout this specification, including the claims which follow, unlessthe context requires otherwise, the word “comprise” and “include”, andvariations such as “comprises”, “comprising”, and “including” will beunderstood to imply the inclusion of a stated integer or step or groupof integers or steps but not the exclusion of any other integer or stepor group of integers or steps.

It must be noted that, as used in the specification and the appendedclaims, the singular forms “a,” “an,” and “the” include plural referentsunless the context clearly dictates otherwise. Ranges may be expressedherein as from “about” one particular value, and/or to “about” anotherparticular value. When such a range is expressed, another embodimentincludes from the one particular value and/or to the other particularvalue. Similarly, when values are expressed as approximations, by theuse of the antecedent “about,” it will be understood that the particularvalue forms another embodiment. The term “about” in relation to anumerical value is optional and means for example +/−10%.

Numbered Embodiments

The following numbered embodiments form part of the description.

-   1. An active electrode material expressed by the general formula    [M1]_(x)[M2]_((1-x)) [Nb]_(y)[O]_(z), wherein:    -   M1 and M2 are different;    -   M1 represents one or more of Ti, Mg, V, Cr, W, Zr, Nb, Mo, Cu,        Fe, Ga, Ge, Ca, K, Ni, Co, Al, Sn, Mn, Ce, Te, Se, Si, Sb, Y,        La, Hf, Ta, Re, Zn, In, or Cd;    -   M2 represents one or more of Mg, V, Cr, W, Zr, Nb, Mo, Cu, Fe,        Ga, Ge, Ca, K, Ni, Co, Al, Sn, Mn, Ce, Te, Se, Si, Sb, Y, La,        Hf, Ta, Re, Zn, In, or Cd; and wherein    -   x satisfies 0<x<0.5;    -   y satisfies 0.5≤y≤49    -   z satisfies 4≤z≤124-   2. The active electrode material according to embodiment 1, wherein    M2 is selected from one or more of Mo, W, V, or Zr.-   3. The active electrode material according to embodiment 2 wherein    the [M1]_(x)[M2]_((1-x))[Nb]_(y)[O]_(z) is a material selected from    the group consisting of:    -   M1_(x)Mo_((1-x))Nb₁₂O_((33-33 α))    -   M1_(x)W_((1-x))Nb₁₂O_((33-33α))    -   M1_(x)V_((1-x))Nb₉O_((25-25 α))    -   M1_(x)Zr_((1-x))Nb₂₄O_((62-62 α))    -   M1_(x)W_((1-x))Nb_(0.57)O_((4.43-4.43 α))    -   M1_(x)W_((1-x))Nb_(0.89)O_((5.22-5.22 α))    -   where M1 represents one or more of Ti, Mg, V, Cr, W, Zr, Nb, Mo,        Cu, Fe, Ga, Ge, Ca, K, Ni, Co, Al, Sn, Mn, Ce, Te, Se, Si, Sb,        Y, La, Hf, Ta, Re, Zn, In, or Cd; and wherein        -   x satisfies 0<x<0.5; and        -   α satisfies 0≤≤≤0.05.-   4. The active electrode material according to any one of the    preceding embodiments wherein the active electrode material is    oxygen deficient.-   5. An active electrode material expressed by the general formula    [M]_(x)[Nb]_(y)[O]_((z′-z′α)), selected from the group consisting    of:    -   MoNb₁₂O_((33-33 α))    -   WNb₁₂O_((33-33 α))    -   VNb₉O_((25-25 α))    -   ZrNb₂₄O_((62-62 α))    -   W₇Nb₄O_((31-31 α))    -   W₆Nb₈O_((47-47 α))    -   wherein a satisfies 0<a≤0.05.-   6. The active electrode material according to any one of the    preceding embodiments wherein at least some of the material has a    Wadsley-Roth crystal structure and/or a tetragonal tungsten bronze    crystal structure.-   7. The active electrode material according to any one of the    preceding embodiments wherein the active electrode material    comprises a plurality of primary crystallites, some or all of the    primary crystallites optionally being agglomerated into secondary    particles.-   8. The active electrode material according to embodiment 7, wherein    the average diameter of the primary crystallites is from 10 nm to 10    μm.-   9. The active electrode material according to embodiment 7 or    embodiment 8, wherein some or all of the primary crystallites are    agglomerated into secondary particles, and the average diameter of    the secondary particles is from 1 μm to 30 μm.-   10. The active electrode material according to any one of the    preceding embodiments wherein the active electrode material    comprises a carbon coating formed on the surface of the primary    crystallites and/or secondary particles.-   11. The active electrode material according to embodiment 10 wherein    the carbon coating is present in an amount of up to 5 w/w%, based on    the total weight of the active electrode material.-   12. An active electrode material according to any one of the    preceding embodiments wherein the crystal structure of the active    electrode material, as determined by X-ray diffraction analysis,    corresponds to the crystal structure of one or more of:    -   MoNb₁₂O₃₃    -   WNb₁₂O₃₃    -   ZrNb₂₄₀₆₂    -   VNb₉O₂₅    -   W₇Nb₄O₃₁    -   W₉Nb₈O₄₇.-   13. An active electrode material according to any one of the    preceding embodiments, further comprising Li and/or Na.-   14. An electrochemical device comprising an anode, a cathode and an    electrolyte disposed between the anode and the cathode, wherein the    anode comprises an electrode active material according to any one of    embodiments 1 to 13.-   15. A use of an electrode active material according to any one of    embodiments 1 to 13 as an anode active material, or a component of    an anode active material, in an anode in conjunction with a cathode    and an electrolyte in: (i) a lithium ion battery for charging and    discharging of the lithium ion battery; or (ii) a sodium ion battery    for charging and discharging of the sodium ion battery.-   16. A method for processing an electrode active material according    to any one of embodiments 1 to-   13 as or in an anode active material for: (i) a lithium ion battery,    wherein the method includes diffusing lithium ions into the anode    active material; or for (ii) a sodium ion battery, wherein the    method includes diffusing sodium ions into the anode active    material.-   17. A method of making an active electrode material according to any    one of embodiments 1 to 13, the method comprising steps of:    -   providing one or more precursor materials;    -   mixing said precursor materials to form a precursor material        mixture; and heat treating the precursor material mixture in a        temperature range from 400° C. — 1350° C. to form the active        electrode material.-   18. The method of making an active electrode material according to    embodiment 17 wherein the one or more precursor materials includes a    source of Mo, W, Zr, or V, and a source of Nb.-   19. The method of making an active electrode material according to    embodiment 17 or embodiment 18 wherein the one or more precursor    materials includes an M1 ion source, an M2 ion source, and a source    of Nb, and wherein the resulting active electrode material is a    material as defined in any one of embodiments 1 to 4, or embodiments    6 to 13 as dependent from embodiment 1. 20. The method of making an    active electrode material according to embodiment 17 wherein the    precursor materials include one or more metal oxides, metal    hydroxides, metal salts or oxalates.-   21. The method according to any one of embodiments 17 to 20 wherein    the one or more precursor materials are particulate materials,    optionally having an average particle size of <20 μm in diameter.-   22. The method according to any one of embodiments 17 to 21 wherein    the step of mixing said precursor materials to form a precursor    material mixture is performed by a process selected from dry or wet    planetary ball milling, rolling ball milling, high shear milling,    air jet milling, and/or impact milling.-   23. The method according to any one of embodiments 17 to 22 wherein    the step of heat treating the precursor material mixture is    performed for a time of from 1 to 14 h.-   24. The method according to any one of embodiments 17 to 23 wherein    the step of heat treating the precursor material mixture is    performed in a gaseous atmosphere, the gas being selected from air,    N₂, Ar, He, CO₂, CO, O₂, H₂, and mixtures thereof.-   25. The method according to any one of embodiments 17 to 24 wherein    the method includes one or more post-processing steps selected from:    -   (i) heat treating the active electrode material;    -   (ii) mixing the active electrode material with a carbon source,        and, optionally, further heating the mixture, thereby forming a        carbon coating on the active electrode material;    -   (iii) spray-drying the active electrode material; and/or    -   (iv) milling the active electrode material to modify the active        electrode material particle size.

REFERENCES

A number of publications are cited above in order to more fully describeand disclose the invention and the state of the art to which theinvention pertains. Full citations for these references are providedbelow. The entirety of each of these references is incorporated herein.

Goodenough and Park, “The Li-Ion Rechargeable Battery: A Perspective”,Journal of the American Chemical Society 2013 135 (4), 1167-1176, DOI:10.1021/ja3091438

Griffith et al., High-Rate Intercalation without Nanostructuring inMetastable Nb₂O₅ Bronze Phases, Journal of the American Chemical Society2016 138 (28), 8888-8899, DOI: 10.1021/jacs.6b04345

Griffith et al., “Structural Stability from Crystallographic Shear inTiO₂Nb₂O₅ Phases: Cation Ordering and Lithiation Behavior of TiNb₂₄O₆₂ ”Inorganic Chemistry (2017), 56, 7, 4002-4010

Montemayor et al., “Lithium insertion in two tetragonal tungsten bronzetype phases, M8W9047 (M=Nb and Ta)”, Journal of Material Chemistry(1998), 8, 2777-2781

Zhou et al., “Facile Spray Drying Route for the Three-DimensionalGraphene Encapsulated Fe2O₃ Nanoparticles for Lithium Ion BatteryAnodes”, Ind. Eng. Chem. Res. (2013), 52, 1197-1204

Zhu et al., “MoNb₁₂O₃₃ as a new anode material for high-capacity, safe,rapid and durable Li+ storage: structural characteristics,electrochemical properties and working mechanisms”, J. Mater. Chem. A.(2019),7, 6522-6532

Yang et al., “Porous ZrNb₂₄O₆₂ Nanowires with Pseudocapacitive BehaviorAchieve High-Performance Lithium-Ion Storage”. J. Mater. Chem. A. (2017)5. 10.1039/C7TA07347J.

1. An active electrode material expressed by the general formula[M][Nb]_(y)[O]_(z); wherein the active electrode material is oxygendeficient; wherein M consists of one of Mg, Cr, W, Mo, Cu, Ga, Ge, Ca,K, Ni, Co, Al, Sn, Mn, Ce, Sb, Y, La, Hf, Ta, Zn, In, or Cd; y satisfies0.5≤y≤49; and z satisfies 4≤z≤124.
 2. The active electrode materialaccording to claim 1, wherein z is defined as z=(z′−z′α) wherein αsatisfies 0<α<0.05.
 3. The active electrode material according to anypreceding claim, wherein (i) M consists of one of Mo, W, Al, Zn, Ga, Ge,Ta, Cr, Cu, K, Mg, Ni, or Hf; or (ii) M consists of one of Mo, W, Al,Zn, Ga, or Ge; or (iii) M consists of one of Mo, W, Al, or Zn.
 4. Theactive electrode material according to any preceding claim, expressed bythe general formula [M]_(x)[Nb]_(y)[O]_((z′−z′α)), selected from thegroup consisting of: MoNb₁₂O_((33-33 α)) WNb₁₂O_((33-33 α))W₇Nb₄O_((31-31 α)) W₉Nb₈O_((47-47 α)) Zn₁₂Nb₃₄O_((87-87 α))Cu₂Nb₃₄O_((87-87 α)) AlNb₁₁O_((29-29 α)) GaNb₁₁O_((29-29 α))GeNb₁₈O_((4-47 α)) W₁₆Nb₁₈O_((93-93 α)) W₅Nb₁₆O_((55-55 α))AlNb₄₉O_((124-124 α)) GaNb₄₉O_((124-124 α)) wherein a satisfies0<α<0.05.
 5. The active electrode material according to any precedingclaim, expressed by the general formula [M]_(x)[Nb]_(y)[O]_((z′−z′α)),selected from the group consisting of: MoNb₁₂O_((33-33 α))WNb₁₂O_((33-33 α)) W₇Nb₄O_((31-31 α)) W₉Nb₈O_((47-47 α))Zn₂Nb₃₄O_((87-87 α)) AlNb₁₁O_((29-29 α)) GeNb₁₈O_((47-47 α)) wherein αsatisfies 0≤α≤0.05.
 6. The active electrode material according to anypreceding claim, expressed by the general formula[M]x[Nb]_(y)[O]_((z′−z′α)), selected from the group consisting of:MoNb₁₂O_((33-33 α)) WNb₁₂O_((33-33 α)) W₇Nb₄O_((31-31 α))W₉Nb₈O_((47-47 α)) wherein α satisfies 0≤α≤0.05.
 7. The active electrodematerial according to claim 1, wherein the active electrode material hasthe formula MoNb₁₂O_((33-33 α)) wherein a satisfies 0<α<0.05.
 8. Theactive electrode material according to claim 1, wherein the activeelectrode material has the formula WNb₁₂O_((33-33 α)) wherein asatisfies 0<α<0.05.
 9. The active electrode material according to claim1, wherein the active electrode material has the formulaW₅Nb₆O_((55-55 α)) wherein a satisfies 0<α<0.05.
 10. The activeelectrode material according to claim 1, wherein the active electrodematerial has the formula W₇Nb₄O_((31-31 α)) wherein a satisfies0<α<0.05.
 11. The active electrode material according to claim 1,wherein the active electrode material has the formulaZn₂Nb₃₄O_((87-87 α)) wherein a satisfies 0<α<0.05.
 12. The activeelectrode material according to claim 1, wherein the active electrodematerial has the formula AlNb₁₁O_((29-29 α)) wherein a satisfies0<α<0.05.
 13. The active electrode material according to claim 1,wherein the active electrode material is expressed by the generalformula [W][Nb]_(y)[O]_(z); optionally wherein the active electrodematerial is selected from WNb₁₂O_((33-33 α)), W₇Nb₄O_((31-31 α)),W₉Nb₈O_((47-47 α)), W₁₆Nb₁₈O_((93-93 α)), and W₅Nb₁₆O_((55-55 α))wherein a satisfies 0<α<0.05.
 14. The active electrode materialaccording to any preceding claim, wherein the crystal structure of theactive electrode material as determined by X-ray diffraction correspondsto the crystal structure of the unmodified form of the active electrodematerial, wherein the unmodified form is expressed by the generalformula [M][Nb]_(y)[O]_(z) wherein the unmodified form is not oxygendeficient, wherein the unmodified form is selected from M2^(I)Nb₅O₁₃,M2^(I) ₆Nb_(10.8)O₃₀, M2^(II)Nb₂O₆, M2^(II) ₂Nb₃₄O₈₇, M2^(III)Nb₁₁O₂₉,M2^(III)Nb₄₉O₁₂₄, M2^(IV)Nb₂₄O₆₂, M2^(IV)Nb₂O₇, M2^(IV) ₂Nb₁₀O₂₉,M2^(IV) ₂Nb₁₄O₃₉, M2^(IV)Nb₁₄O₃₇, M2^(IV)Nb₆O₁₇, M2^(IV)Nb₁₈O₄₇,M2^(V)Nb₉O₂₅, M2^(V) ₄Nb₁₈O₅₅, M2^(V) ₃Nb₁₇O₅₀, M2^(VI)Nb₁₂O₃₃, M2^(VI)₄Nb₂₆O₇₇, M2^(VI) ₃Nb₁₄O₄₄, M2^(VI) ₅Nb₁₆O₅₅, M2^(VI) ₈Nb₁₈O₆₉,M2^(VI)Nb₂O₈, M2^(VI) ₁₆Nb₁₈O₉₃, M2^(VI) ₂₀Nb₂₂O₁₁₅, M2^(VI) ₉Nb₈O₄₇,M2^(VI) ₈₂Nb₅₄O₃₈₁, M2^(VI) ₃₁Nb₂₀O₁₄₃, M2^(VI) ₇Nb₄O₃₁, M2^(VI)₁₅Nb₂O₅₀, M2^(VI) ₃Nb₂O₁₄, and M2^(VI) ₁₁Nb₁₂O₆₃, wherein the numeralsI, II, III, IV, V, and VI represent the oxidation state of M.
 15. Theactive electrode material according to any one of the preceding claimswherein at least some of the material has a Wadsley-Roth crystalstructure and/or a tetragonal tungsten bronze crystal structure, orwherein substantially all of the active electrode material has aWadsley-Roth crystal structure and/or a tetragonal tungsten bronzecrystal structure.
 16. The active electrode material according to anyone of the preceding claims wherein the active electrode materialcomprises a plurality of primary crystallites, some or all of theprimary crystallites optionally being agglomerated into secondaryparticles.
 17. The active electrode material according to claim 16,wherein the average diameter of the primary crystallites is from 10 nmto 10 μm.
 18. The active electrode material according to claim 16 orclaim 17, wherein some or all of the primary crystallites areagglomerated into secondary particles, and the average diameter of thesecondary particles is from 1 μm to 30 μm.
 19. The active electrodematerial according to any of claims 16-18 wherein the active electrodematerial comprises a carbon coating formed on the surface of the primarycrystallites and/or secondary particles.
 20. The active electrodematerial according to claim 19 wherein the carbon coating is present inan amount of up to 5 w/w%, based on the total weight of the activeelectrode material.
 21. The active electrode material according to anypreceding claim, wherein the active electrode material has a BET surfacearea in the range of 0.1-100 m²/g, or 0.5-50 m²/g, or 1-20 m²/g.
 22. Anactive electrode material according to any one of the preceding claimswherein the crystal structure of the active electrode material, asdetermined by X-ray diffraction analysis, corresponds to the crystalstructure of one or more of: (i) MoNb₁₂O₃₃ WNb₁₂O₃₃ W₇Nb₄O₃₁ W₉Nb₈O₄₇Zn₂Nb₃₄O₈₇ CU₂Nb₃₄O₈₇ AlNb₁₁O₂₉ GaNb₁₁O₂₉ GeNb₁₈O₄₇ W₁₆Nb₁₈O₉₃ W₅Nb₁₆O₅₅AlNb₄₉O₁₂₄ GaNb₄₉O₁₂₄; or (ii) MoNb₁₂O₃₃ WNb₁₂O₃₃ W₄Nb₇O₃₁ W₉Nb₈O₄₇Zn₂Nb₃₄O₈₇ AlNb₁₁O₂₉ GeNb₁₈O₄₇; or (iii) MoNb₁₂O₃₃ WNb₁₂O₃₃ W₇Nb₄O₃₁W₉Nb₈O₄₇
 23. An active electrode material according to any one of thepreceding claims, further comprising Li and/or Na.
 24. Anelectrochemical device comprising an anode, a cathode and an electrolytedisposed between the anode and the cathode, wherein the anode comprisesan electrode active material according to any one of claims 1 to
 23. 25.A use of an electrode active material according to any one of claims 1to 23 as an anode active material, or a component of an anode activematerial, in an anode in conjunction with a cathode and an electrolytein: (i) a lithium ion battery for charging and discharging of thelithium ion battery; or (ii) a sodium ion battery for charging anddischarging of the sodium ion battery.
 26. A method for processing anelectrode active material according to any one of claims 1 to 23 as orin an anode active material for: (i) a lithium ion battery, wherein themethod includes diffusing lithium ions into the anode active material;or for (ii) a sodium ion battery, wherein the method includes diffusingsodium ions into the anode active material.
 27. A method of making anactive electrode material according to any one of claims 1 to 23, themethod comprising steps of: providing one or more precursor materials;mixing said precursor materials to form a precursor material mixture;and heat treating the precursor material mixture in a temperature rangefrom 400° C.-1350° C. to form the active electrode material.
 28. Themethod of making an active electrode material according to claim 27wherein the one or more precursor materials includes a source of M and asource of Nb.
 29. The method of making an active electrode materialaccording to claim 27 or claim 28 wherein the precursor materialsinclude one or more metal oxides, metal hydroxides, metal salts oroxalates.
 30. The method according to any one of claims 27 to 29 whereinthe one or more precursor materials are particulate materials,optionally having an average particle size of <20 μm in diameter. 31.The method according to any one of claims 27 to 30 wherein the step ofmixing said precursor materials to form a precursor material mixture isperformed by a process selected from dry or wet planetary ball milling,rolling ball milling, high shear milling, air jet milling, and/or impactmilling.
 32. The method according to any one of claims 27 to 31 whereinthe step of heat treating the precursor material mixture is performedfor a time of from 1 to 14 h.
 33. The method according to any one ofclaims 27 to 32 wherein the step of heat treating the precursor materialmixture is performed in a gaseous atmosphere, the gas being selectedfrom air, N₂, Ar, He, CO₂, CO, O₂, H₂, and mixtures thereof.
 34. Themethod according to any one of claims 27 to 33 wherein the methodincludes one or more post-processing steps selected from: (i) heattreating the active electrode material; (ii) mixing the active electrodematerial with a carbon source, and, optionally, further heating themixture, thereby forming a carbon coating on the active electrodematerial; (iii) spray-drying the active electrode material; and/or (iv)milling the active electrode material to modify the active electrodematerial particle size.
 35. The method according to any one of claims 27to 34 wherein the method includes a post-processing step of heattreating the active electrode material in an inert or reducing gaseousatmosphere at temperatures of above 500° C. to form oxygen deficienciesin the active electrode material.